There have been a few short sections where the tunnels were enlarged (and of course most of the original City and South London from near Borough to Stockwell), but that tends to be for specific reasons of tight clearances.
I'm sure in 1930s there used to be engineering trains, overnight park in tunnel, remove some upper ring sections, pare back the clay, reinsert sections with extra cast iron section (about 20cm wide). Don't get a perfectly round tunnel, but did ease some clearances on curves. Was also used on Central Line when platform tunnels extended to ease the gradients (as line was built with inclined tunnels approaching and leaving platforms to aid acceleration and deceleration). Even today on most Bakerloo, Piccadilly and Central Line platform tunnels can see a kink in platform lining where platform tunnel extended around the original running tunnel.
On many 1930s extension sections, stations have platforms designed for 8 or 9 standard cars so are longer than trains. Some 1930s relocated stations like Aldgate East also built with longer platforms.
But not worth trying to increase tunnel diameter throughout, much easier to build new line.
However there may be a case for platform extensions to be resumed, for instance many Piccadilly line platforms are now longer than the 6 car trains, but only makes sense if trains are being replaced. Unlikely to be full width, but sufficient to reach last door without causing congestion. Some of these extra bits of platforms are now fenced at front to allow DOO monitors and mirrors, but latest trains have moved this to in cab screens so could be reused with new trains. Maybe increasing frequency to 36 tph is cheaper than small platform tunnel extensions, as either way have to buy extra carriages.