No, the tube is not much bigger, it is thin wall steel... about 1/4" [6.3mm].
Your reference is a secondary source... I have the original Elco torpedo tube drawings. The actual inside diameter of the breech collar is machined to 21.41 inch... 27.2mm in 1:20 scale... actually slightly smaller than the (approximate) tube outside diameter I mentioned below.
Regardless, these parts are sized to fit 3/4" PVC pipe, which is commonly used by modelers for 1:20 scale PT boats.