Qt5 is slightly heavier than GTK+2 in their experience, but lighter than GTK3. Since GTK2 is dead and won't be ported to Wayland, LXDE has to move to /something/. Having to choose between Qt5 and GTK3, they picked Qt.
Qt is not that heavy, BTW. Depends on what you use, and LXDE will only use the minimum they need.