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I would guess the reason is to speed up the combustion as in a large bore cylinder it takes time for the flame front to travel from the center to the sides which gives the unignited charge more time to heat up and a risk of detonate.

To go from a used 125xc to a used 250xc is probably going to be over $1k. You might be able to sell the 125 xc and get an older used yz250 and break even.

The hole I see in your argument is that you want to sell a used 125, then buy a used 250 and have money left over for a truck. Just how much do you think you're going to get for your clapped-out 125XC? (That's not a dig at your maintenance skills, just an assessment of what every buyer everywhere is going to think - they'll offer you $1500 cash for that bike. "But it's CASH!!") I feel like if you break even on the swap from a 125 to a 250, you will have done well.

taking in account your age and the doubts you refer as to your ridding technics my advice to you is, invest a bit in yourself, do a couple of good ridding schools, get an idea of where you are at and set good targets to aim your ridding evolution.......then worry about the best bike to suit you.....now you have a great learning bike.... dirt bike ridding is always more rider then bike......

250X’s rock