The 1st/2nd numbers, respectively, correspond to the amount of hydrocodone in the pill (the first number), and the second number is the amount of APAP (Tylenol) in the pill. The 7.5/750 not only has 2.5 more mg's of hydrocodone than the Vicodin, but it also has 250mg's more Tylenol. So, the extra tylenol makes the pill bigger (2.5mg's of powder would not make a substantial difference in size, but 250mg's *would*) - So it's not really bigger because it is stronger per se, but bigger because it has more Tylenol mixed in with it.