Copper Pennies

How come 1980-1996 weigh less? if they are all 98% copper shouldn't they weigh the same? This is what I was confused on because I found a couple weighing 3.2 grams then the ones that were 2.5 I didn't keep cause I did not think it was copper.

So I've found a couple Canadian Pennies that say 1867-1892 for the date. Are these 98% copper too?

Information from Charlton catalogue.

Content;
1937-42 = .955 copper, .030 tin, .015 zinc. 19.05 mm. 3.24 gm.
1942-79 = .980 copper, .005 tin, .015 zinc. 19.05 mm. 3.24 gm.
1980-81 = .980 copper, .005 tin, .015 zinc. 19.00 mm. 2.80 gm.
1982-96 = .980 copper, .005 tin, .015 zinc. 19.10 mm. 2.50 gm.

Thickness; pre 1965 ?
1965-79 = 1.55 mm.
1980-81 = 1.38 mm.
1982-96 = 1.45 mm.

After 1996 they are copper-plated zinc(non-magnetic) or steel(magnetic).