Well, what could I say? These coins are great. This "R" after the year shows they were made in Rio de Janeiro. Now the reason why one is "bigger" than the other I can´t tell you with 100% sure.
I should say that´s because coin-making was imprecise, but it´s just a guess, I´m sorry.
I have a Brazilian coins catalog here and it doesn´t say anything specific about these coins.
If I find any other information, I post here.