Little Bams are hellishly modified Piglins, having undergone some sort of magical transformation to warp them into their current form.
Given the realm of the Nether that they live in, it's not clear exactly what process caused the transformation, but it's believed to be some form of natural occurance.

Their diminutive stature gives them a quicker speed, and their warped form gives them a very noticeable instability that seems to relentlessly torture them until their final release.
When this does occur, they prefer to do so in style, with a sizeable explosion released from some sort of internal power buildup.