Yes you can always go heavier gauge. Mowers tend to be made on the very edge of acceptable wiring to make them function when new, but add corrosion and those almost too small wires can create issues.
Otherwords OEMs are squeezing Abe Lincoln until he is screaming in pain. As the bare minimum wire gauge cost less than the next size up. Just if going up in size it require different terminal if are not just splicing in a section.
