Personally files this large become just very annoying overall. Code review tools tend to become slow on them (e.g. Gerrit is super-slow with files that are quite a bit smaller than qwidget.cpp), navigation is only possible via symbols, the import / include area becomes a huge mess (which also tends to generate constant merge conflicts, because imports are a common hot area for writes) etc.
Personally files this large become just very annoying overall. Code review tools tend to become slow on them (e.g. Gerrit is super-slow with files that are quite a bit smaller than qwidget.cpp), navigation is only possible via symbols, the import / include area becomes a huge mess (which also tends to generate constant merge conflicts, because imports are a common hot area for writes) etc.