carry on back, horse load
馱 centers on bearing a load: carrying something on one's back, or a horse's burden. The idea is supporting and transporting weight.
馱 combines 馬 (horse) with 大 (big), suggesting a horse carrying a large load. The exact historical development is uncertain, but the component relationship is straightforward.
A horse (馬) with something big (大) on its back: it's carrying a heavy load.
For タ, imagine a horse carrying a tall stack of luggage: tall -> タ, and the horse bears the load.