Seems like there isn’t a strong definition for “megafauna.”
In zoology, megafauna (from Greek μέγας megas "large" and Neo-Latin fauna "animal life") are large animals. The precise definition of the term varies widely, though a common threshold is approximately 45 kilograms (99 lb), with other thresholds as low as 10 kilograms (22 lb) or as high as 1,000 kilograms (2,200 lb).
Not a strong definition but still sufficient to exclude squirrels and even wild pigs are on the very low end.
You raise a good point as I would have expected “mega”fauna to have implied hundreds of kilograms at least. Under 100 and as low as 10 kg seems to take away any true meaning from the label.
In zoology, megafauna (from Greek μέγας megas "large" and Neo-Latin fauna "animal life") are large animals. The precise definition of the term varies widely, though a common threshold is approximately 45 kilograms (99 lb), with other thresholds as low as 10 kilograms (22 lb) or as high as 1,000 kilograms (2,200 lb).
https://en.wikipedia.org/wiki/Megafauna