This very interesting article explores a few possibilities to the sauropod mystery, and comes to the conclusion that the atmosphere was much thicker in the mesozoic era, allowing for extra buoyancy and less gravitational pull :

"To produce an effective buoyancy force on

dinosaurs the Earth's atmosphere would have to

be thick enough to have a density comparable to

the density of water. By summing the forces

acting on a typical dinosaur such as a

Brachiosaurus the density of the necessary

atmosphere is calculated as:

?F = ?S (1 - 1/S.F.)

where ?F is the density of the fluid, ?s is the

density of the substance submerged in the fluid

such as the dinosaur, and S.F is the scaling

factor.

Inserting into this equation a scaling factor

of 3.2 and an overall vertebrate density of 970

kg/m3, the Earth's atmospheric density during

the late Jurassic period can be calculated to be

670 kg/m3.

This says that to produce the

necessary buoyancy so that the dinosaurs could

grow to their exceptional size, the density of the

Earth’s air near the Earth’s surface would need to

be 2/3’s of the density of water.

It may be difficult for some people to imagine how

the Earth could have had such a dense

atmosphere. But nevertheless, the wonders of our

reality often exceed the limitations of many

people’s imagination. Esker’s Thick Atmosphere

Theory violates no property of science. It is the

correct solution."

Quite convincing

www.dinosaurtheory.com/solution.html