I'm on guix 563ab27bd4caead5676bbd515c54b25100c98960 on guixsd.
$ guix package -I; contains
lilypond	2.19.80	out
/gnu/store/jhmz7wpb93hryzqq32kcb4fs9cd84jyi-lilypond-2.19.80
I installed lilypond with that guix version.

$ guix size lilypond; gives me
guix size: error: no available substitute information for
'/gnu/store/6zyrglcmk8l4xpwvg6hgq1al81d7fr1w-lilypond-2.19.80'

I have no idea how this happens.
A wild guess is that store/jhmz.. is a grafted version, and guix size
looks for the ungrafted one?

Martin
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GPG: 7FDE 7190 2F73 2C50 236E  403D CC13 48F1 E644 08EC