Not all generations are of the same size. The iPod Touch 2nd and 3rd Generation are nearly (if not completely) identical in physical dimensions and appearances AFAIK. The height, width, depth are all the same, the weight is the same. The only difference is the increase in RAM and a slightly better processor. An analogy for the iPod Touch 2nd and 3rd gen is like the iPhone 4 and the iPhone 4S. The physical appearance is near identical with respect to the front and back (not including the aluminum bezel since that's the method of differentiating a 4 and the 4S), so cases are generally tagged "iPhone 4/4S"
The iPod Touch 4th Generation is slightly slimmer, slightly lighter, but also slightly longer. Cases are differentiated between iPod Touch 1st Gen, iPod Touch 2nd/3rd Gen, iPod Touch 4th Gen, and the iPod Touch 5th Gen, so there are essentially 4 tiers of classification for cases with respect to the iPod Touch. If you see a case that says "iPod Touch 2nd/3rd/4th Gen", unless it's a fairly loose/flexible soft case, it won't be compatible with all 3 generations.