This is a common issue with Compressor fridges, people look at the consumption figures and choose a cable size accordingly but that doesn't allow for the much higher current drawn momentarily when starting up. Sounds like yours is right on the cusp of being adequate. Increase the cable guage, you may get away with doing just the pos or neg only if you do just one and retry it but bear in mind things get worse as the
battery voltage gets lower so probably best to build in a safety margin and do both.