Removing linux-firmware from the build [Devel Digest, Vol 65, Issue 52]

[Yioryos Asprobounitis]

 > The reason is that df is known to not produce good results
> on jffs2
> nor on ubifs (but ubifs is supposedely much more truthful).

I do not know why you say that when the data show otherwise. 
(and although I can not find the post right now, I remember that James Cameron came on a similar conclusion - df being fairly accurate - in some tests of his own)

I also not clear to me how random data can be different on average, over an extended length.

Maybe you could investigate yourself in a way that you find more reliable.
100MB is considerable space for the XO-1.