First let's go over what malt liquor is. Brewing yeast is able to eat most but not all of the carbohydrates present in the wort (unfermented beer). In order to get more alcohol for less money many brewers have tried to drive this up. One way of doing this is by adding enzymes to the wort that break down the carbohydrates present so that the yeast is able to eat more. This makes the drink drier and more alcoholic without requiring more grain. It also makes the drink taste thinner and have less flavor. So beer with these enzymes added (which are pretty much the same thing as beano) is what malt liquor is. That's why it's stronger and cheaper than regular beer. Lite beer is exactly the same thing as malt liquor just watered down. It is "lite" because the added enzymes allow the yeast to consume more carbohydrates which means less carbohydrates left for you when you drink it.

This process of adding enzymes was first done during the Great Depression to save money. Later on the marketing teams of breweries tried to convince people that malt liquor was an upscale alternative to champagne. This is why many of the older brands of malt liquor have upscale names such as "Olde English." This marketing campaign directed at upscale champagne drinkers didn't work very well but, seemingly by accident, the breweries noted that they were getting surprisingly good sales of malt liquor in African American neighborhoods.

The introduction of 40oz bottles didn't happen until the 80's, well after breweries started setting malt liquor. A lot of malt liquor marketing at the time was based on dancing around alcohol advertising laws that prohibited advertising that trumpeted alcohol content as a reason to buy the drink and selling malt liquor in bigger bottles seemed to be part of an effort to convince people that malt liquor was a good way of getting really drunk.

Unsurprisingly there isn't much in the way of academic research about malt liquor. However this Thrillist article seems to be well researched to me with a lot of effort put into tracking down various bits of information as well as some hilarious malt liquor advertising posters from the 50's: https://www.thrillist.com/drink/nation/malt-liquor-history-sleazy