In my understanding, 125s corner faster because the 250f's weigh more and the extra weight tends to be somewhat high on the bike, meaning you have a higher center of gravity. This is the primary reason I do not want a 4 stroke (well that and I love the 2 stroke hit). I'm 5'4, 145 and I HIGHLY dislike even my bros KX250 2 stroke. The extra weight up high makes it feel like a tank to me.
Don't even begin to think your bike is too old - its an '04. I started racing in June and I'm on a 2002 RM 125. I'm in the top 3 in Open D every week, mid pack in Unlimited C/D and mid to upper pack in C. Many times I beat newer 250f's. I've even passed them on long, steep uphills. However, in a close race, I do have trouble passing a 250f coming out of a corner - followed one for 4 laps and I was never further behind than 10 feet, but never got past him. I'm hoping the Eric Gorr 144 kit with mo better porting will help come springtime :)
One of my buddies is on a 1998 KX 250. On a very tight local track where he is actually at a disadvantage because of the weight of his bike and the low speeds, he still beats me by a good 5-10 seconds. He's just plain better than me regardless of the bike.
TAKE A CLASS!! I cut probably 2 seconds a lap off my times within 2 races of taking a class from a local who finished 8th at Loretta Lynn's this year. Worth every penny to have someone watch and correct . . .
Also, to get you started now, try this email course:
http://www.motocrosstrainingcenter.com/
Or this book:
http://www.amazon.com/gp/product/07...103-1864501-3579811?n=507846&s=books&v=glance
Rob