FYI. I found Unity to have and overhead of apx 7-10mb depending on required features.
Technically you can make a Chrome Wrapper and cut out features. For example if your not using WebSockets you don't need to include them. Same with DOM rendering and other such features. However the only way to cut these features to reduce the size is to manually cut the Chrome features and pack it yourself. None of the automated Chrome Wrappers I notice offer this feature. So I'm sure you can reduce the size, but your technical knowledge is above most C2 demographic knowledge.
Again I must itterate Unity's own engine size almost clicks in at 10MB. So is really 7mb all that big a deal?