Milchkühe, why so small?

[Gudestein]

Guess this is more of an amateurs technical question on the atlantic part of the WW2, so hope its in the right forum.

The germans had anticipated to fight cruiser wars against french and later British ships from 1938, and had seen the ability to refuel their raider at see as a critical issue. They must have known they would be vulnerable.
What I am wondering is, do we see any technical reasons why submarines could not have been massively enlarged as fuel carrying vessels. The fuels could have been carried outside the pressure hull. Sure, it would be clumpsy, but its defense would be concealment.
There was a purpose build merchant sub (Deutschland) of about 1500 tons that could carry a cargo of 700 tons of solid materials. I guess it could be 50/50 or more in favor of fuel load.
WHat was build was a scaled up type IX submarine with a 25% of total weight as its load for refuelling type IIV's.

Why wasn't a big clumpsy tanker not not seen as the ideal solution to keep Bismarck and Tirpitz in the Atlantic?