Today, I'd say there are a few cases where page size REALLY matters and they all relate directly to total load time.
Disclaimer: I'm not taking about unnecessary large page size due to incompetence, poor optimization, etc. I am also speaking within the context of average North American internet speeds.
1) majority of your users are mobile and likely to not be on wifi when using the site 2) testing shows that you are losing a noticeable percentage of users/conversions due to long load times 3) user base uses old tech (i.e. government comps running windows xp)
Otherwise - 3mb downloads in seconds on most connections. There are numerous programming techniques to minimize load time of larger files; modules, conditional loading, Ajax etc.
Gone is the day of web developers squeezing out the "just one more KB" on a website. (This should and is still true for libraries and such but not really for whole websites)
Like I said though, this is for average American internet. Every userbase dictates a different requirement for page size - but the majority of sites in 1st world countries can be 6mb without a huge issue.