Can't answer the Matilda question, Michael, but as for Bertha, there were at least three 'Big' ones, the gun you mention, an English traction engine, and an American steam organ. To the anglophone, big and Bertha seem automatically linked.

I had a customer, a German lady who was named Bertha, who pronounced her name Bee-ar-ta (last 'A' almost a click) rather than the English style Ber-tha.
She wasn't big, either. Very trim.