OK, how does this sound:
I got my DT250 when I was 14. Rode it a LOT, and even convinced myself that I was pretty good on it - everytime I went to a riding park, I'd regularly blow away plenty of the weekened warriors on their moderns.
A few years later, I got a modern 125 and rode it until I was actually pretty good on it in the bush (but knew I was ordinary on a MX track).
Much more recently, I went out and bought myself a couple of 'proper' VMX bikes: A YZ250G and a YZ125D. Rode both around at a mate's farm, and figured that I had the 250s measure and that the 125 would just be a bit of fun.
When I actually fronted up to a race meet with them, the 250 beat me half to death (and I was always finishing at the back of the field), while the 125 was fun right to the end of the race and I was finishing mid-pack.
I've been racing semi-regularly for about 3 years now, and I now know that I lap faster on a 125 than on my 250.
What happens on a race track in a real race is quite different than any sort of rec riding - even play racing.
Don't go thinking that racing a 125 will be boring - for me, the pressures, the fitness and the concentration needed while racing mean that I can't race a 250 properly. The lower powered 125 requires a fair bit less concentration to ride, giving me more time to race (if that makes sense).
Similarly, on my 125s, I can often place pretty well in the 250 races - so I suspect that the above is true for lots of people, and not just me.
I'm not trying to talk you out of the 250, just making sure that you go in with your eyes open, and not simply follow the age-old mantra that says "If you're big, then you need a 250".