Editing skins and tyres?

[Cheesehead2]

Hi, I'm just taking my first steps at editing skins: I'm just adapting some existing ones to my own tastes, probably the hard way, i.e. directly editing the skin.dds file with paint.net, a nice free software I'm familiar with. Even so, I can't complain about the results

I'm curious to know, though, why the edited file is always smaller than the original, even if I save it with the same resolution, color depth and compression?

Also, I would want to edit some tyres (changing or removing the tyre brand logo), but no idea how to do that. Would someone please let me know?

Thanks in advance, best regards.