Yeah, thats normal. One major factor is that drag is related to the square of airspeed, ie 2x the speed requires 4x the power and 4x the speed requires 16x the power. So if full throttle generates 2x the thrust of half throttle, the airspeed only increases by the square root of 2, or 1.414.

Add to this the fact that even running static on the ground most engines don't have a truly linear rpm curve related to the throttle position.

As has been pointed out, the best solution to this (if you have a computer radio) is to set up an exponential curve on your throttle channel.

-Jeremy