But also: Soundwave eating properly for the first time at the gladiator school and bulking up and being able to support multiple symbionts at once!!

Exactly!

And then there’s the flipside of that. Personal headcanon: symbiotes are beneficial to the host, in part because they improve the circulation of energy. A larger frame supports more symbiotes, which, in turn, help the host to grow and support thick armor. 

When Soundwave starts losing his symbiotes to the war, their former docks die for lack of energy being pulled through them, then the extra energy pathways and armor attachments start to die as well. That takes him back to his former lanky build, his frame no longer able to support that much thick plating, and his stress level doesn’t go low enough for long enough to kindle another symbiote.