Notonly is there a big difference in the peak output, there is a huge difference in the way the power is put out. A 125 is a high-rpm screamer, you have to rev the snot out of them to get the power. You have to learn good clutch skills to ride a 125 with any sort of speed. A 250 makes a lot more power a lot sooner in the rpm range, and pulls over a broader spread, with a lot more torque available. They need a lot less clutch work, and a lot less revs to get the job done.