I'm not an expert on Mexican coins and there is no need for one: all information is there on the Numista coin page.
In the year lines it clearly states that there is a light variant (“thin flan”, 10gr, 1974-1977, KM#477.1) and a heavy variant (“thick flan”, 14gr, 1978-1985, KM#477.2).
So the OP's heavy 1981 coin is exactly what it should be.
N#1072