SpiderMonkey in Gecko is probably about 3.5 million lines of code. Gecko, the web engine that SpiderMonkey is a part of is about 12 million lines I think, and the front end for Firefox brings the total near 20 million lines. This is all dated info from when I last looked maybe a decade ago but I think it's directionally correct and roughly similar to Chrome.
So, one might save about 35% or so of the browser's code by stripping out JS and any other bits that are there exclusively for JS in the DOM and elsewhere.