If you work with large amounts of data it hugely matters. 16 bytes times 1 billion records ends up being way more than 16GB of data, indexes, foreign keys, hot backup, etc. to manage and query.
Even that is a small amount of memory. I have that much RAM on my computer. See, its an obsolete conversation, yet folks raised in an ecology of scarcity can't get past it.
It depends on what you are optimizing for. If you need to store your entire database in physical memory, then the difference between 16GB and 128MB is significant.