Push ups are great to build up strength and endurance, but won't really give you significant mass increase, unless you add some weight to your back. I do many push ups myself. A lot of variety to target different muscles, and I also add 65 lbs in a backpack.
The trick is to find ways to make the push ups harder. You can also try rolling push ups, which basically consists of using 2 dumbbells(That can slide/roll). This requires immense balance and strength.
I agree with the above. 40 push ups isn't a lot. If you have been doing push ups for 2 years, you should easily be able to do 100-200 push ups within a 30 minute period.