It's working correctly. When changing animation frames, the sprite changes size automatically to scale to the next frame. For example if you switch from a 100x100 frame to a 200x200 frame, the sprite becomes twice as big so it looks correct. In other words, when changing between animation frames, the sprite size is scaled by the scale factor of the two animation frames.
When you set the object to 64x64 it hasn't changed frame yet. Then it changes frame and the Sprite plugin notices it's going from an animation frame about 100x100 to 250x250. This is about 2.5 times bigger. This scale is applied to the object (currently sized 64x64) and it ends up with the final size around ~160x160.