Even with those chemical changes in the body, the woman's ability to pass something 7lbs proves that the human vagina can be very elastic. Your Asian chick example isn't even necessary because any guy who has ever nailed a virgin, even with an average sized cock, already knows how much pain a woman can have when she's too small (to start with) for what she is getting penetrated by. Stuffing your junk through an unbroken hymen takes serious work! She's very sore after the first time, sore each subsequent time for a couple weeks, and then her vagina opens up enough over time to where there is no more problem. If you day after day ram a 9 inch length 7 inch girth dick into a woman (who has already had sex dozens of times before), her box will stretch out over time and eventually be able to take a monster cock like that without any problem.
Not to go off topic but this is just ridiculous. The chances of finding a woman who is ok with 'being rammed with a 9x7 day after day for weeks til she stretches' are slim to none. The chances of starting PE and having a woman who is ok with you saying 'I'm going to make my penis much bigger, so big that it will cause you excruciating pain, but its ok cause if I bang your brains out for a month you'll adapt and it'll only be very painful rather than extremely painful' are just 0.
If you are with the one partner and they think you are the perfect size, what possible gain is there in a bigger penis? You would have to care more about what stranger in locker rooms think of your penis than how enjoyable your partner finds sex. All you're doing is making things less enjoyable for her, requiring more lubrication, more foreplay, slower strokes, you're limiting what/how you can both have sex.
As long as we love each other, start slowly, pay attention to the other person's needs, and use plenty of lubricant, I really don't see how another .5" in length and .25" in girth is going to make much of a difference.
It's not like I'm shooting for 9x7 (at least not yet)
.25" is a noticable girth increase, 6" can be very problematic for some. I would definately advise staying away from girth work until you've had sex, you're alread 5.75" girth which is huge. No one can deny that and you should start realising that, but its really, really huge. My girlfriend would not be able to accomodate that, not at all, it simply would not go inside of her and she would not want it to try.

