Get yourself along to 20th Century Chap: http://www.20thcenturychap.com/ They have some lovely 1930's fish tail trousers up to 40 inch waist. And their jackets go up to 46 inch. Warning, their website is exceedingly moorish, you could easily damage your wealth.I've found the occasional 44 jacket over the years. Nothing much bigger.
As for reasons, previous generations of people were smaller. The relatively recent increase in human height has been linked to improvements in childhood nutrition.