I understand exactly, I have a couple of planes that perform that way. Like above, the servo operates in a linear fashion, but throttle response may not be linear. In other words you may get almost full throttle performance with the stick at only half, and the last half of stcick travel doesn't seem to do much, and you don't have as much fine control at lower throttle settings. You can improve this by messing with both ends of your end point adjustment on the throttle servo. I usually set mine up by starting with 100% throw on the servo, I set the stick at half, so the servo is at half of its total throw, and then connect the linkage so that the carb barrel or butterfly is about 1/4 open. Start it up, and then adjust endpoints to get a fully open throttle and a low enough idle.

Some radios have expo for throttle channel. My 9C Futaba has it, and you can make an exponential curve that will give you linear throttle response. My DX-7 does not have expo, one of the few things I don't like about it.