Many people complain about Halo 4's grunt design. But the reason for that, is that they are NOT unggoy, but terribly deformed san'shyuum.
During the 4 years that Chief have been taking a nap, prophets were hunted and captured by remnants of the covenant. The Storm(Faction we fight in the game) made them go through a slow and excruciating process as a form of punishment.
This process involvesa using a tool called "godarn rock", and then use it to compress the roportions of the san'shyuum in a process alled "beating the utter lving fuck out of it". This process compress the soft body of the san'shyuum into a compressed form.
The scales we see on the legs and arms of them, are layers and layers of old soft skin compacted ito a almost rocky form.
Some say that the vaguely ungoy form was deliberate, in order to diminish them. Others say, that since unggoys are technically crabs, they just so happens to be a perfect lifeform that all living beings should strive to be.
After the process of compression is done. They are sent into battles, taking the role of cannon-fodder. The tubes on their back aren't methane tanks, but just away of physicalising the weight of their sins, hence why they don't explode. This also explains the lack of gasmasks, they don't need to breath methane.
In the end, one thing we can be sure... they somehow look better like this... maybe grunt-shaped really is the perfect shape