It's a matter of the escape velocities of the atmospheric constituents and chemistry.
Both Venus and Earth have similar surface gravities and hold on to similar gases in the same way. Once those gases are present in their atmospheres there is no major mechanism apart from EXTREME heat ( causing the molecules to exceed escape velocity) or chemical reactions ( by locking up the gases in the surface rocks) that can reduce the atmosphere.
Mercury is just too hot to hang on to any atmosphere.
Venus was born with a heavy CO 2 atmosphere and little of it has been lost.
Earth probably had a similar CO 2 load but it has been locked up in the surface rocks, the oceans and in plant life which Venus does not have.
Mars, with its low escape velocity, has a thin CO 2 atmosphere due to its low gravity allowing most of its original atmosphere to bleed into space.
If our poor Earths atmosphere continues to accumulate CO 2 it will simply continue to do so until we have unlocked all the carbon in the rocks and oceans and then we will start to look like Venus!
It's not an "air grasp'" difference. Both planets are the same.Both hold on to CO 2 the same way.
The difference is in the initial CO 2 present and the way in which the CO 2 has been removed from the atmosphere.
Another effect is temperature of course. Venus being closer to the Sun and hotter will tend to loose the lighter gases more than CO 2, hence the higher concentration.
CO 2 is a "heavy" gas and tends to stick around.
It is all a little more complex than this when we have to consider the effect of UV light and other processes on the evolution of an atmosphere, but gravity is not the important aspect.