When to use yo versus mí

Can you help me understand why "yo" is used in this sentence instead of mí? I thought that yo is used when it is the subject of the sentence. Isn't the majority of the friends the subject in this sentence instead of the speaker? Thanks!

Most of my friends are taller than me. La mayoría de mis amigos son más altos que yo.

submitted by /u/huskeractuary
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