It's all about the cube-square law, the same reason a butterfly can remain airborne for hours or days on a few milligrams of chemical energy.
Getting RC aircraft to fly in the more laborious, sedate manner of a full-size aircraft instead of like an insect fluttering through the wind is something of a challenge for scale model builders. When you build an airframe around tiny batteries as a design constraint, expectations from larger aircraft don't carry over well.
Getting RC aircraft to fly in the more laborious, sedate manner of a full-size aircraft instead of like an insect fluttering through the wind is something of a challenge for scale model builders. When you build an airframe around tiny batteries as a design constraint, expectations from larger aircraft don't carry over well.