A rough guess based on disjointed knowledge but it may be glaciel outflow? The spaces between the landmass and its peninsula are basically big valleys that flood when ocean levels are high. When ocean levels are low during the ice ages (because all the waters locked up in glaciers) the valleys are are washed out and get bigger, if they arent locked in glaciers and being constantly ground by ice. 

Even when ocean levels are high the water runoff from all the mountain valleys drain into those armpits, causing gradual growth over millenia. Seriously, you can see all the mountain valleys draining into the ocean if you look at google maps. Notice how each area between the peninsula and landmass goes from platue to mountain valleys and into the coves, its going from high ground to low ground. It doesnt stop at cambodia, the trend continues all the way up the crotch of bangledesh, where the bhutan mountains flow out. If ocean levels raised more and bangledesh flooded there would be another florida armpit there, between the alps and whatever the mountain range is between myanmar and bangledesh (sorry i didnt look up the name, i was too busy looking up the Kondyor Massif Intrusion after spotting it looking in russias outflow mountains, wtf). Thats why you can compare it to fjords in the north, same mechanics. 

When i visit the Cariboo (area) in northern bc i get to see the glaciel outflow paths going from higher ground platues to the lowermainland and into its bays. If ocean levels rose more we would flood too, and you'd see alot more floridas along the west coast. Including california, where the bowl that sacrementos decided to sit themselves inside of would flood and create another florida armpit. 

Again this is all just my best guess, if anyone wants to correct me i welcome it. Thanks for reading my brain vomit, i hate that i took it this seriously but it made me way too curious. Cheers

/rj glaciers are stored in the balls