Yes that is somewhat normal for all engines that use a carb with a throttle plate. It varies quite a bit though depending on the size of the carb venturi compared to the size of the engine. You can resolve the issue by adding differential to the throttle linkage and / or by using a throttle curve in your transmitter. There has been much written about this here and in other forums. A few searches should get you more info than you ever wanted to know.