Louise,
I think maybe you mean to say it's 7.5/750 you have there, which is generic Vicodin ES. (Personally, I'd feel that's unfortunate for you, Vicodin ES is my least favorite short acting med, only because it's the one that comes with the most tylenol per pill). I'm not sure how many a day you're taking, but be careful, as you don't want to take more than 4000mgs of tylenol in 24hrs (well taking that in a day won't kill you obviously, but doing that much continuously will harm your liver, or even taking as much as 3500mgs or so continuously too). So, as a rule, you really only want to be taking 5 of those a day at most to be in a safer range.
And you may have heard this before, and you'll most certainly hear it again - everyone reacts differently to everything (in response to your ? about being knocked out). I used to work with a guy, he is 6-1, about 190 and two regular Vicodins would have him pretty much down for the count. I'm a mere 5-9, 160, and they have the reverse effect on me, they actually kind of energize me.
And finally in regards to why you're so tolerant, that's completely dependant upon how long you've been on these meds, which that I don't think you've mentioned. Obviously the longer it is, the higher tolerance you have.... Aye there's the rub, when it comes to opiate analgesics, there's no ceiling to the dosage. Hope this helps