I have a few of the 1/1400 models and they do 'fit' with Tri-angs. I think the larger scale for ferries is possibly because:

1. - they fit the same size package or space on their sales carousels, or:
2. the size is a critical factor in how a child perceives the 'play value' of a toy. A small toy inevitably has lower 'play value' for a child.

Corgi did much the same thing with its model cars in the 1970s. They jumped in size from around 1/43-1/48 scale to 1/36 (I think it was). A bigger 'handful' of toy was considered by Corgi to be more attractive to children.