Can't see anything wrong there and it loads into mach fine here. Roughly 54mm square.

If you can't find any obvious reason ie Step per Settings etc then I'd install Mach3 again seen daft things like this happen before. just install over top of existing setup all settings will be retained.

Sure this won't be the case but you haven't set scale factor have you.? If you have the scale LED's will be flashing at you so will be obvious.