Why Are Nickels Bigger Than Dimes?

Why are nickels bigger than dimes, I can't stop pondering 
I've begun this letter a thousand times, my mind keeps wandering 
I should say it was great to see you 
I'm so glad you had a wonderful time 
You made home seem very near but darling I 
Don't think you should come back here and okay here's why 

It's not your fault, you just remind me 
Of the life I'm trying to forget 
Until I put it all behind 
I'm asking, no I'm pleading will you let 
Me go somewhere I've not been 
I bet you know exactly what I mean 
Because you said yourself it's like we're walking off on two completely different paths 

Why are nickels bigger than dimes 
Why are nickels bigger than dimes