When studying the internals of Postgres years ago, I learned that pages could greatly affect the disk space required to hold a table. If the page size was 8192 bytes (the default) and if the schema defined each row as holding 4097 bytes, then two rows would not fit within a single page. This would cause every row to be within its own page and would waste almost half of the space. Anyone know if this is still true?
What you are describing is called "internal fragmentation" and it's always a problem at some level in any system.
There are tons of ways to mitigate the problem including variable-length data, out-of-line storage, and compression. Postgres does those things, but I suppose there's always room to improve.
Best to just see how much storage a given table uses, and see if it's a problem.