I've had the opposite experience. Happily it doesn't happen often.
I was going to Montreal not long after 9/11. By the time I flew traffic had dropped so BA merged the Montreal flight with a Boston and we went London/Boston/Montreal. Essentially downgauging!
But, sometimes I wonder a little. I once had a trip Gatwick Nantes. Evening flight out, night in Nantes, meetings, evening flight back. When I arrived at Gatwick the flight was cancelled and they put me in the Hilton and flew me on the flight next morning. Even with the extra load from the previous evening that Embraer wasn't full. I found myself wondering how full the inbound and outbound flights had been the previous evening.