Listen to what BigD said. He is right on point. I am not the only one that will tell you this. I have rode quite a few 125's and 250's. My brother has two 125's. I also own a KDX200 probably the best trail bike you can get. One of my sons owns a 250f and it is a nice trial bike as long as you don't kill it(four strokes suck to restart) but If you like shifting a lot and working the dog piss out of the clutch, then by all means go with 125. I have nothing against them. I think they make better riders out of most...especially someone learning, but what I don't like is all the shifting all the time. I can be in the wrong gear on my 250 by one gear and still power out of a situations. With the 125, you'll have to shift.
Like biggieD said the they are basically the same weight and they are the same size. Google the specs on the two RM125 and RM250. Let me tell you this, if you think the 250 is heavy...good luck on that 400 four stroke
that is the weight of two 250's!