Learn French: Je viens de m'arrêter parce que je m'essouffle plus que toi. - I just stopped because I get out of breath more than you.

Yes, you pronounce the "s" (sounding like /plys/). When plus modifies a verb to mean "more" (here, comparing how much you get out of breath), the 's' is pronounced. This is different from when plus modifies an adjective that starts with a consonant (like plus fort), where the 's' is completely silent.

Why does the sentence end with toi instead of tu?

In French, you must use a stress pronoun (such as moi, toi, lui, or elle) after the word que in a comparison. Because you are saying "more than you," the standard subject pronoun tu must be swapped for the stress pronoun toi.

Why is it m'arrêter and not s'arrêter if it's an infinitive?

Even when a reflexive verb remains in the infinitive form (here, because it follows the viens de structure), the reflexive pronoun must still agree with the subject of the sentence. Since the subject is Je, the pronoun changes from the dictionary form se to me (which contracts to m' before a vowel).

How is the verb s'essouffler built?