My understanding is that game cartridges are all standard sizes (32, 64, 128), but not all games need to use all that space, so the leftover space can be trimmed. Certain ROM dumping tools will automatically trim that padded space, which I believe makes the file smaller. Overtrimming is when the tool trims too much space and causes the ROM to malfunction (which might be anything from freezing to not working at all).
A 'proper' ROM should retain the cartridges' original size, padding the extra space with zeros. Certain dumping tools will ensure that a ROM is never overtrimmed, but the person who dumped their copy didn't use those tools (it was the first time they ever tried, apparently).
edit: Just to relate to this particular game, we know what the cartridge size is supposed to be, because we already have a clean (proper, untrimmed) ROM of the Japanese version. The ROM that was dumped is a different size, it doesn't have the zeros that padded the Japanese version.