Would guess that as per always this depends on the route and flight number. YYZ-BOS, YYZ-LGA, YYZ-ORD have all been relatively empty on my recent flights. In contrast to your experience these are all routes that were packed pre-Christmas / pre-eupgrades and are now empty.

I think it has simply emphasized the routes that are 'haves' with a high demand for upgrades (more so than before) and routes that are 'have-nots' or 'nice-to-haves' with overall fewer upgrades.

Effectively, everyone is conserving for the really high value upgrade flights which are experiencing higher than previous levels of competition.