The Lobel-Page edition of Alcaeus consists of 4,888 words, while that of Sappho consists of 3,459 words (these figures are from the TLG). That's the fact, give or take; the interpretation is less straightforward,* if any can or, indeed, should be made. It depends both on chance survival of what happened to be in circulation in small set of locations and on deliberate, but selective, preservation.
So, more of Alcaeus has come down to us from Antiquity, whether on papyrus or as extracts and fragments, even of only a single word, that suited the needs of grammarians and literary critics.
Yet, editions of the two 'Great Lesbians' usually put Sappho before her (allegedly older) contemporary and add 'et SAPPHUS vel ALCAEI FRAGMENTA' 'et SAPPHO et ALCAEUS'. There is, as far as I know, no translation of Alcaeus alone, while there are several of Sappho alone.
* What, for example, counts as a word? A fully preserved lexical item, for sure, but what about partially preserved words that can be reasonably supplemented and what about a single letter or letter trace as the sole residue of a word?
Both are mentioned by Herodotus (Alcaeus, briefly, at 5.95.1-2 and Sappho, and her family, at greater length).