Theoretically it can be both, but most probably the particular sentence is understood as the more humans (the subject) eats, (the subj.) gets stronger.
It is the usage of
ほど
4〔「…すればするほど」の形で〕
行けば行くほど道は狭くなった
The further we went, the narrower the path became.
見れば見るほどいやになった
The more I looked at it, the less I liked it.
早ければ早いほどいい
The sooner the better.
In some cases ・・れば part can be omitted, like 早いほどいい. The sentence is understood as 人間を食えば食うほど強くなる just because it sounds plausible as a description of fictional monster; the other interpretation implies eating a man indicates strength, which is unlikely.
As another example, 普通の食事ができるほど元気になった means (The subject) recovered to the extent that (the subj.) can eat normal food. I have the impression that ・・できる・れるほど tends to fall on this interpretation.
- 話せるほど大きくなった grew up to be able to speak
- ペーパーバックが読めるほど英語がわかるようになった became good at English enough to read paperbacks.
- (Compare: using ペーパーバックを読むほど here sounds the more I read paperbacks, the better I became at English.)