[Eric in the Ozarks]

The locks had and have little to do with the lack of international size ships transiting the lakes. 28 foot draft might be more of a reason. Plus the Welland Canal limits lake shipping to about 75 feet width.

[Alberta's Child]

All true. But there was a time when a 28-foot draft was probably sufficient to accommodate the ships that were in use for much of the history of North America. I believe the current configuration of the St. Lawrence Seaway dates back to 1959.