I happen to be poking around the Chromium codebase the last few days. The size of the codebase itself is at the same level as all of our company's code. Something as important and critical as GPU rendering is only a small part of the entire project. You also have v8, ChromeOS, ANGLE etc to worry about, all requiring experts in those areas. Not to mention things like Widevine and other proprietary technology surrounding Chrome.