I'm not with you, on my L720 live tiles are really small when add the third row.(my opinion)I quite like it. Feels weird when I turn that off.
I have an 8x and I think its fine and my son has a 520 which also looks fine but would depend on what your preferences are to things you think are small. I'm taking it you have a 620 if so why not just look at someones start screen on wpcentral and view the 3 row and see for yourself.