I have found that it seems every onesie sizing chart is different. It seem difficult to determine the correct sizing from the charts as few give you measuring instructions. Also you need to know if you should be wearing a diaper while measuring or if there is an allowance build into the chart. Sizing charts could be in men's or woman's cuts which cloud the water even more. A man may have to go up one to two sizes and a woman would have to go down one to two depending on the chart.
I have bought onesies from Kins (Babykins), ABU, and a friend. The Kins one fit quite well for the most part but depending on the fabric not all mediums fit the same. They have since discontinued some of the fabric choices and the ones they sell now are similar in fit even after washing. ABU oneies fit longer and I might have been better off getting a small instead of a medium. But they have elastic around the legs that give them a different look and fit. I don't mine them fitting longer since they look a little more like a romper than onesie when I wear them. ABU is going to add regular fit to their sizing charts which will give you a choice of a shorter body length. The one I bought from my friend is a XL which was most likely sized on a women's sizing chart as it fits the same as my other ones.
The popping snaps sounds like one of two things, bad snaps or the size is too small. If you wash the onesie it will most likely shrink a little, sadly this may make the problem worse. If you keep the one you have you could try replacing the snaps with better ones from a sewing store. It is not the easiest job to do and the fabric could suffer a little.
