What applications are preinstalled is completely irrelevant. An application that is installed but not running costs nothing but disk space.
I don't know why you feel that elementary must be heavier than Gnome. Gnome itself is pretty heavy.
There was an article comparing Linux desktop environments in the Layer 3 Networking Blog in April 2013. Elementary didn't exist yet but the author tested a lot of environments. While figures can vary quite a bit depending on what applets, widgets and so on are loaded, the order of magnitude is telling: ~200MB for KDE, slightly less for Unity and Gnome3, ~50MB for lightweight desktop environments, a few MB for heavyweight window managers, <1MB for lightweight window managers. (Those figures are for the WM/DE only, not the base system.)
Hectic Geek compared elementary Luna with Gnome in August 2013. They found that there was no significant difference in memory usage and boot times.
Brendan Ingram compared the RAM usage of several configurations in July 2016. He found that Gnome on Ubuntu 16.04 took 700MB (that's total RAM, not just the desktop environment) while Pantheon on the elementary OS version based on 14.04 took 600MB, i.e. Pantheon used slightly less memory but that was an older version.
The upshot is that Pantheon as configured by elementary and Gnome or Unity as configured by Gnome use similar amounts of memory. Elementary's default setup requires roughly the same amount of resources than Ubuntu's.