I can't really attest to the reasoning behind the stats for maces. Hatchets and maces have one crit for each ton they weigh and someone wanted maces to weigh more, which is reasonable and consistent (even if I disagree with it).
They could have tweaked the mace to have fewer crits per ton--but still more crits than a hatchet for the same type of 'mech. I suppose Battletech's...tumultuous relationship with fractions is the reason.
As an uneducated guess. "Must weigh more than hatchets. Must have as many crits as tons. Nothing over 80 tons being able to carry it is acceptable."
You can always house rule it, but if I make those suggestions, this thread might get moved again.
Supplementary issues:
-Sudden jump in crits from 100-ton 'mechs to 105-ton 'mechs.
-Universal size of engine crits, regardless of engine rating.