Consider a table that you know will only have a few rows, but is referenced in one or more FK columns. If I use a 64bit integer when 32bit (or smaller!) is enough, I’m now using twice as much space for the FK column. If there are millions of rows, plus indexes, that can add up pretty fast.
Yeah, this was a case of somebody (definitely not me, I would never!) generating a table in Rails, before Rails used 64 bit IDs on primary keys by default, and no one noticed before the table got really large. It looks like newer versions of Rails are doing it by default now: http://www.mccartie.com/2016/12/05/rails-5.1.html
Consider a table that you know will only have a few rows, but is referenced in one or more FK columns. If I use a 64bit integer when 32bit (or smaller!) is enough, I’m now using twice as much space for the FK column. If there are millions of rows, plus indexes, that can add up pretty fast.