I used to go touring on a motorcycle with just a small back pack, then along came girlfriends etc. As soon as you get your romantic life sorted out you discover it comes with a lot of baggage. This created a requirement for; first saddle bags, then hard panniers, top case tank bag etc. Then all the above with a sidecar.
With small cars you start with the boot, then overflow to the back seat, finishing with the boot full and a roof rack to carry everything ultimately moving to a station wagon.
Don't even get me started on the Morgan. Suffice to say we leave for a weekend with enough for a month and come back with most things that weren't even unpacked. Plus there is an overflow of "shopping".
My theory is that luggage mysteriously expands to fill the space available, and, there is never enough space available