We are adding a 4 pixel padding to every object, so they dont bleed into each other when we generate the sprite sheet. The sprite sheets are also always generated in power of two increments to support low end hardware.
The 4 pixel padding is something that we show in PSD Importer, and maybe something we should bring to the Aseprite Importer as well, so that users like yourself could bring down to 0 if you know you do not require that additional padding.