Looking at the ideal gas law, p·v = R·T (v is the specific volume, the inverse of the mass density), I'd say beside the pressure p, i.e. the weight of the atmosphere above a point, it's at least the temperature T that is quite different between Earth and Venus. An other factor is the compressibility of the fluid; our air is quite compressible, the atmospheric pressure is quite low and the ideal gas law thus gives a good approximation.

As an atmosphere consisting of multi-atomic fluids, on Venus mainly CO_2, in an ~~over~~ supercritical state is far beyond what can be considered to be an ideal gas (low pressure, single-atomic gas), there are several other factors to be taken into account. The van der Waals law or the Clausius law are usually models to choose when dealing with real gases. However, as mentioned, the atmosphere is in an ~~over~~ supercritical state, i.e. beyond the critical temperature and pressure, the fluids are not in gaseous state, thus an even more complex material model is needed, to describe its behaviour.

But to give an idea of the effect: When you measure the pressure and the density of the water of an ocean in different depth, you'll notice the pressure increases linearly with depth, while the mass density of the water remains almost constant. Its low compressibility is why water often is treated as an 'incompressible' fluid. Similarly, this may work in an ~~over~~ supercritical fluid, as it has some properties that are similar to gases and some, e.g. lower compressibility, that are more similar to liquids.