Blame the Americans - in 1784
They wanted to be their own boss, it all got a bit messy sending soldiers there on big ships and guns had only just been invented and everybody wanted one. It all got a bit costly, so a clever Lord somebody whispered a jolly good idea in someone else's ear. They all thought about it for a bit, realised it wouldn’t impact the Aristocracy too much, and whopped a 2s6d (2 shillings and sixpence - which was quite a lot back then) tax on every thousand bricks made.
Sizes?.... Oh yes..the naughty manufacturers started making bigger bricks to avoid the tax! They had 66 years to play sizes with the tax man. The higher the tax the more cost effective to make bigger moulds.
The tax man eventually gave up and went away but things didn't get properly sorted out until decimalisation in the 1960’s because we had so many different sized moulds kicking around.