I need to know! Surely they're not half the length of all other lines because the platforms are full length...
The main reason for many Bakerloo line trains having an extra driving cab car in the train somewhere is because the current carriages/trains were not originally purchased for that line. The current stock is (refurbished) 1972 train sets which were Northern Line stock.
Northern line trains were a carriage shorter
This page gives a longer overview of the historical rolling stock of the system.