They are high-poly for entities being pushed to the graphics card every frame. When you use RotateEntity its only pushing a small amount of data (probably a matrix or similar), whereas RotateMesh pushes all the vertex and triangle data - which is huge.
Its a seperate issue, but polycounts depend on many things: type of game, target hardware, style of graphics, distance from camera, number of surfaces (at least in Blitz3D), use of alpha/masking etc. ie. very hard to pin down.
Its a seperate issue, but polycounts depend on many things: type of game, target hardware, style of graphics, distance from camera, number of surfaces (at least in Blitz3D), use of alpha/masking etc. ie. very hard to pin down.