Now I see the difference.
Ok, my opinion is by making logo bigger, that editor overrote some data from the logo, which was used for background attributes. Since smb doesn't have free space by the looks of it, I have no idea how editors suppose to edit anything in the game if user wants to make something bigger.
If I wanted to make this hack by myself, I would find an existing hack for converting smb to another mapper with more free space (or made one myself), then transfered logo data to a new free space and manually edited it anyway I want to. But you said this is your first hack, and I guess you don't know assembly at all, so this might take too much time to explain how to do this.
Maybe I'm wrong, and your editor can do such magic things, but I can't help you with that. There should be several editors for smb (after all, this is freaking mario we are talking about), perhaps you should try another one.