OK, I'm going to build on @alex-b's answer, having looked at Lobel-Page 90 and Campbell 90. Those more game than me can look at the original papyrus: http://22.214.171.124/gsdl/collect/POxy/index/assoc/HASH01f6.dir/POxy.v0021.n2293.a.01.hires.jpg
Sean Palmer of http://inamidst.com/stuff/sappho/ has done a great service in putting Sappho online, and something of a disservice in printing the line as he has.
Sean Palmer has reproduced what we are now calling "the mishmash" of Sappho quotes, at the start of his "Wharton 12 / Cox 12" fragment (which is Lobel-Page 26). The fragment MickG could not trace is [ε]γω το καλλος επιτ[μεζον]…
That's not what the fragment is.
The fragment comes from Lobel-Page 90. Lobel-Page is a commentary on Sappho, and Lobel-Page were reluctant to put spaces into such a small fragment; that's why I couldn't find the words when searching. They supplied a guessed reading in their app crit. The passage in question is (col iii 18-19):
In their app crit, they suggest this might ("fort.") be read as ἔ]γω τὸ κάλλος ἐπειτ.[ |μέζον· τὶ γάρ ἠνεμ[
ἐπειτ.[ means the line breaks at [, and there's an unreadable letter before it. | means new line.
There is no word ἐπιτ[μέζον]: it's a dord. And wherever the mishmash has come from has completely misread the app crit. The passage is ἐπιτ- ... μέζον.
Campbell offers the translation "'I... beauty... greater'; for what was...". He also reads μέδον instead of μέζον, but he emends it to μέ<σ>δον, which is the same thing anyway.
Because there's a mention of "I", we assume this is a verse of Sappho, and not the commentator's text. (Campbell in his translation ascribes 'I... beauty... greater' to Sappho and "for what was" to the commentator.) We can't really guess what the verb starting with ἐπιτ- is. The mishmash translation as "In my life I served beauty" is... well, I don't know what it is based on. I also still don't know who to blame the mishmash on; it's not Lobel–Page, Campbell, or Cox, and I'd be astonished if any classicist would have the chutzpah to smash all those fragments together.