Earth and Venus have a very similar gravity, but the mass of atmosphere on Venus is much greater (according to this wikipedia article 93 times larger). I know that the chemical composition and temperature are different, but there is just a lot more matter in the Venusian atmosphere. Why is this?
It is thought that about 4 billion years ago, Venus had an atmosphere and oceans similar to Earth's. Owing to it's proximity to the sun and the fact that most of its primeval atmosphere was carbon dioxide, it heated up and the oceans evaporated away, causing more greenhouse heating as they did so. Water vapour is a potent greenhouse gas.
Earth's primeval atmosphere was also mainly CO2, but being about 30 million miles further away from the sun it did not experience the runaway greenhouse effect which overheated Venus. The seas did not evaporate into the atmosphere, as they did on Venus, where the water mixed with sulphur dioxide/trioxide to create clouds of sulphuric acid droplets.
Another thing which has thinned the Earth's atmosphere is billions of years of sequestration by living things, which have transformed atmospheric CO2 into limestone (CaCO3), other carbonates, and various forms of fossil hydrocarbons (Lange et al., 1983). The photosynthetic organisms which produced the hydrocarbons also produced huge quantities of oxygen, which today makes up 21 percent of our atmosphere.
If you could magically turn all the carbon sequestered away in the rocks back into CO2, and evaporate our oceans so that they went up into the atmosphere, the Earth would be much more like Venus, including the runaway greenhouse effect which has heated Venus's lower atmosphere to 460C. Being much further away from the sun than Venus, Earth would not heat up as much as that, but it would still be too hot for life.
Mars also had an atmosphere similar to Earth's and Venus's, but because of its much weaker gravitational field has been unable to hang onto them. However, the proportions of the gases in the Martian atmosphere are still similar to those on Venus, though far more tenuous.
Lange, M. A. & Ahrens, T. J.(1983):"Shock-Induced CO2-PRODUCTION from Carbonates and a PROTO-CO2-ATMOSPHERE on the Earth", LUNAR AND PLANETARY SCIENCE XIV, P. 419-420. http://adsabs.harvard.edu/full/1983LPI....14..419L
The facile answer is "Venus's atmosphere has many times the mass of Earth's atmosphere because it has many times the amount of gas". Which makes us ask wonder why that is.
Many planetary scientists have interpreted the relatively even distribuion of impact craters on the surface of Venus as a sign that the planet was resurfaced in some catastrophic event 500-700 million years ago:
From the craters visible in Magellan's Venus maps, scientists believe they are looking at a relatively young planetary surface, perhaps about 500 million years old. Since Venus formed at the same time as Earth 4.6 billion years ago, some event or events 500 million years ago must have resurfaced the planet. Scientists believe that this may have been the work of massive outpourings of lava from planet-wide volcanic eruptions.
-NASA, Magellan Summary Sheet.
The initial estimate of 500 million years has since been pushed back to a bit older, around 700 million years. (Earth was in a snowball phase about this time).
Whether the event on Venus was purely volcanic or the result of an impact is still an issue of debate. Regardless, this was a volcanic event many orders of magnitude greater than the Siberian Traps whose gases caused the end-Permian extinction on Earth.
Earth's atmosphere is replenished from volcanic outgassing. We can speculate that the catastrophe released an enormous amount of gas into Venus's atmosphere.
Between 700 and 750 million years ago, a near-global resurfacing event triggered the release of carbon dioxide from rock on the planet, which transformed its climate.
(Wikipedia: Life on Venus)