Short answer: it depends.
Long answer: older OSs are technically smaller in their "footprint": they take up less space on the drive, they take up less RAM to run, & they don't require as much system resources. However, this is also countered by the hardware involved. For example, assuming I could even get Windows 7 to run on the first PC my wife & I bought (which was a Socket 7-based system rocking a 300MHz AMD K6, a 32GB HDD, & IIRC about 512MB of RAM by the time I had to replace it), I would shudder to think about the boot-up time it would have. But that was also back when you had a "fast" home connection if you had a 56K modem, & would swoon if you could surf the web over a T1 connection at school or work, & most home PCs were running Windows 95 or 98. When XP came along, I'm sure it would have run as slow as molasses on the old PC...but the PC that replaced it (Athlon XP 3200+ system) had a lot more juice, so it could run XP much more easily than the old hardware could.
The tradeoff, however, is in compatibility & security. As much as possible, newer OS versions try to maintain as much compatibility as they can so that older software doesn't immediately go belly-up. But that compatibility means the new OS has to not only maintain the old code, but also add in all the newer features...which is why a lot of older Windows 98 stuff, let alone Windows 3.1, stopped working with Windows XP & hasn't a prayer of working with Windows 7/8/10. By the same token, the newer the OS, the better the security features & updates which are missing from the older versions.
So, if you want to keep an old XP machine for those games that dont' work with Windows 7/8/10 (& aren't available in an updated format through Steam or GOG), or even dual-boot on your machine, go ahead...just make sure that when XP is running, it's not connected to the Internet.