Depends a lot on the end user; what's lightweight? In terms of download speed, those still under .5 Mbps might only consider browsers under 10 MB, while those with 1Mbps+ might be satisfied with 30MB (which opens up a lot of possibilities).
Moreover, the experience you're looking for is also very subjective. I myself can't stand the browsers under 3 MB (including deps), simply for their lack of support for very common features (i.e. CSS, JS, SSL, plugins, and... Flash!). This is definitely not the case for some, who can live entirely off text, going so far as to abstain from images, (HTML formatted) tables, lists, etc.
All depends on your point of view.