First off, not all dinosaurs were giants. They ranged in size from that of a small birds (a few hundred grams) to the 30-40 tonne monsters. Also, the extremely large forms only existed at certain times during the age of dinosaurs. The Late Jurassic to Early Cretaceous seems to have been the time when the 30-40 tonne forms existed. A review of dinosaur body sizes compared to mammals showed that, on average, dinosaurs were 10 times heavier than the average mammal. We also have to remember that the fossil record is most likely biased towards preserving large, robust bones, and biased against small, delicate ones. So our sample of dinosaurs is skewed towards larger forms. There are two lines of evidence to show that dinosaur gigantism can be accommodated within a view of the past that is compatible with present day biology, physics, and our knowledge of the Earth and its evolution – a uniformitarian view.
By applying ideas from engineering to the study of bones, it has been shown that the limb bones of dinosaurs were more than strong enough to resist the bending, compression and torsional forces that they would have experienced due to the weight of the animals – weights that arises from the product of their estimated masses and the present day gravitational acceleration at the surface of the Earth (9.81 m/s^2). Another factor is that large dinosaurs had very erect limb postures that minimized the bending and torsional loads that they would have experienced. Large animals also move more slowly. Slower motions mean smaller accelerations to the limbs and body, and the forces and resulting stresses are greatly reduced. The slow walking speeds (1-4 kmh) of large dinosaurs can be estimated from their preserved trackways. We see all these strategies – strong bones, erect posture, slow movements – in living elephants. Elephants don’t jump. They can’t, and would break bones if they tried. It would have been the same for very large dinosaurs.
It has been observed that the largest dinosaurs only existed during times of super-continents. The northern one, Laurasia, consisted principally of Eurasia, Greenland and North America. The southern one, Gondwana, consisted of Antarctica, Australia, Africa and South America. These super-continents existed during the Jurassic and very earliest Cretaceous. Check this map of the world at that time. When these super-continents began to break apart during the Cretaceous, the 30-40 tonne monsters vanish. The largest dinosaurs we find in the Late Cretaceous are on the order of 5-10 tonnes. It would seem that large areas of terrestrial habitat are needed to house and feed sustainable populations of 30-40 tonne dinosaurs. The modern world is characterized by lots of smaller land masses, hence the smaller average size of the terrestrial faunas. The last hurrah for exceptionally large land mammals was during the last ice age, when the northern continents (the largest ones) hosted large mammoths.
I know of no one who promotes the idea that the Earth has dramatically increased in mass of the past 200 million years. Yes, there are the many tonnes of rock and dust that the Earth accumulates every day, but this is a tiny fraction of the initial mass of the Earth when it first formed. From the Physics Factbook: "The Earth gains mass each day, as a result of incoming debris from space. This occurs in the form of meteors and space dust. The actual amount of added material depends on each study, though it is estimated that 100,000,000 kilograms of in-falling matter accumulates every day. This seemingly large amount, however, is insignificant to the Earth's total mass and represents only one quadrillionth of one percent to its weight each day." [Editor: one quadrillionth is 0.0000000000000001 or 10^-15 plus the percent adds two more zeros. Mass in the form of photons from solar radiation would be billions of times smaller than this, hence even more insignificant, not to mention that most of it is reradiated or reflected back to space.]
There are also arguments from the geological record of tidal deposits from 400 million years ago (time well before that of the dinosaurs). After correcting for the slowing down of the Earth rotation due to viscous drag caused by the oceans and atmosphere, the timing of these ancient daily tides match the expected values for the orbital periods expected for an Earth-Moon system with essentially the same masses that they have today. We have no evidence to suggest that mass of the Earth has significantly changed since the time of the dinosaurs.
Gravity is one of the weakest of the fundamental forces. It would take extremely large amounts of new mass to be added to the Earth to significantly change the gravitational force at the Earth’s surface. Another argument that was once promoted was that the ‘G’ – the universal gravitational constant – was lower in the past, so dinosaurs would have had it easier. Maybe in the earliest times of the Universe G was changing, but not in the past 200 million years.
On the subject of flying dinosaurs, first off, pterosaurs are NOT dinosaurs. They are very closely related as both are members of a larger group known as archosaurs, but there are distinct anatomical differences. Crocodiles are also grouped in with the archosaurs.
The largest flying bird today is the Kori bustard, and this can have a mass of up to 18 kg. It does seem to struggle to take-off from the ground, and may be at the limits for what is possible for powered flight in birds.
With one exception (see below) all known pterosaurs have been estimated to have weighed not more that 18 kg. All our knowledge about the atmosphere of the past indicates that it was not substantially different from what it is today. However, there have been suggestions that oxygen levels may have been higher in the past, and this may have influenced the evolution of various animals. Our present knowledge of flight in living organisms suggests that pterosaurs would have been competent flyers. They were around for 160 million years, and their remains are found on all continents - they must have been doing something right.
The largest known pterosaur is Quetzlcoatlus northropi. Unfortunately, this animal is only known from a few scrappy remains. Mass estimates have ranged from 64 kg to 440 kg. Until we have better knowledge of this animal, we ought not to speculate on what it could, or could not, do. This is the safest and most scientifically honest answer at present. Palaeontology is a waiting game. You can always expect new and better fossils to appear in the future. It is a waste of time to speculate about scraps.