The size of the circulation isn't the only thing that effects the wind speed. There's many factors that go into it. Larger tornadoes are generally stronger, but not always.
Elie MB is one case where the tornado getting smaller towards the end actually was the reason it was so strong (conservation of momentum) becoming Canada's only F5 to date. And a very Canadian one at that! Turned over a semi truck, damaged an industrial building, then threw one house into the air (F5 damage), damaged some others, no injuries. Also had a goofy looking path:
Updated tornado track of Elie, Manitoba 2007 F5Wikiwillz, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
