As someone who has actually written (albeit following a tutorial, for a hobby project) a working non-UEFI bootloader, and having done a lot of experimentation with dual-booting, manipulating disk images, etc.:

I find it hard to believe dropping legacy boot support saves hundreds of megabytes of any data in the OS, let alone source code. (If anything, the UEFI code is orders of magnitude larger / closer to that size, than legacy boot support.)

I could believe that dropping legacy boot support drops a lot of complexity and arcane, old code that's no longer as well-understood. However the actual footprint of that code can't be large. The boot sector is only 512 bytes, then you chain to a secondary and maybe tertiary step in the boot but those are measured in kilobytes or megabytes, hardly hundreds of megabytes (compiled).

But the fact is once the boot process has completed there's no longer any reason to refer back to any of the stuff that happened during the legacy boot while the operating system is running normally; by that point how the boot was accomplished is a non-issue. So it's not like, for example, supporting outdated/obscure hardware where you have to keep a bunch of drivers around.