We don't know.
I believe the closest thing we have to a contemporaneous account about Theodoric is Jordanes' Getica. It was written about 100 years after Theodoric died. He does cite as his source a previous supposedly more extensive work on the Goths by Cassiodorus, however all copies of that work are lost, so all we know about it is the little bit Jordanes chose to give us.
Getica interestingly has a whole page of description for Atilla*, but only this one sentence description for Theodoric (as translated by Charles Christopher Mierow):
He was a man of the greatest moderation and notable for vigor of mind and body.
While not the detailed physical description you were probably hoping for, we can probably surmise that the man should not be portrayed as either overweight or scrawny. At least not according to Jordanes.
* - The description of Atilla was clearly intended to paint the man as the villain of the piece. Its only missing a "Boo! Hiss!" cue card. Which makes the action-hero-esque description of Theodoric a bit suspect too. But its what we have to work with.