Glibc isn't GPL - it's LGPL which allows the creation of closed-source commercial userspace apps.
On a system with shared libraries, the LGPL imposes virtually no requirements on the app's licensing. With static linkage, there are requirements but they aren't all that onerous and don't require disclosure of app source code.
So if the GPL is being used as a reason to avoid Glibc, that's spurious nonsense. Somebody is peddling FUD.
If size is the reason - very likely - why didn't they use uClibc like a lot of small embedded systems use? That's LGPL too, but tiny compared with Glibc.
And if uClibc wasn't ok - why not newlib which is the other commonly used free library for these things?
(I say this as someone who did write my own libc for a big application once, so I'm not entirely unsympathetic.)