That's a very, very relative statement.
They are also a team that likes to have control over the ball, which following the same logic, you can do better on a bigger pitch. When you compare how often they'd have to press vs how often they'd have to lead the game with possession (assuming that most teams that will visit them at Wembley will have less possession than them), a bigger pitch should actually, theoretically, benefit them.
Also, WHL is really not that smaller than Wembley compared to other grounds.
Here's a visualisation - Wembley, WHL, Emirates
They are most certainly going to be affected by the switch of stadiums (quite possibly negatively) but it's mostly down to psychological reasons that come with change in general.