It is better to compare the 100 to the 85. The 125s are such completely different bikes. The HP ratings, which are always a little dubious are something like this: KX 85 - around 18-20 hp, the KX 100 - around 21-23 hp and the 125s are in the low to mid 30s. My experience is somewhat different than Vic's, if you want the KX 100 to fly (and it will) we've found that you still have to rev the snot out of it.
The 100 is like an 85 on steroids, stronger everywhere, but especially in the lower rev range, this can be easily accentuated with some low-tension reeds. The wheels are also bigger than the 85, I believe they are 16" back and 19" in front.
I think your height would be a good fit but at 145lbs you might be a little heavy for the bike's suspension and power output.