Couple of bullet points to clarify for people:
- These guys aren't Spartans... yet. Locke for sure becomes a Spartan-IV by the events of Halo 5, which are voluntarily recruited and not forcefully abducted as kids like the IIs and IIIs. The MJOLNIR armor adds on a few hundred pounds and a bunch of extra height depending on how big the suit is, exactly.
- The Spartans undergo some physical augmentations that affect their actual bodies, for example "carbide ceramic ossification" that makes their bones nigh-unbreakable. On top of this, there's muscular enhancements and reaction time improvements to actually be able to "pilot" the MJOLNIR armor, which is by all accounts a human-sized mech. Without physical augmentation, even the strongest UNSC space marine would crumple up like waste in a trash compactor due to how advanced the power suits are. While some trickery might go on to make sure Locke doesn't have to dope up to look Spartan buff during Nightfall, he's going to get even beefier in-universe when he gets recruited into the Spartan-IV program.
- Human weapons in the Halo universe have become relatively lightweight in contrast to their size for a few reasons: in particular, a lot of their arms are "stripped-down" because the Covenant ate up a lot of their budget in the past. On top of this, while the weapons look huge in human hands, they're meant to be proportional to the size of a Spartan wielding them. Even in past games, the guns look huge in the hands of infantrymen but pretty reasonable when wielded by Spartans.
- The Forerunners had advanced technology pertaining to "natural, not artificial" atmospheric generation and climate control and also specialized in technological compartmentalization to avoid getting trade secrets stolen if anything fell into the hands of their enemies. Chances are the Halo Array, some of their most notable pieces of tech, have the capacity to engage in individualized meteorological stability, even if the ring is fragmented. Note that in-game the metallic techy "motherboard" portion on the outside of the ring repeats and tesselates every so often over the course of the entire circumference of the ring, so it's likely that they produce "pizza-cutter" portions of the ring each with identical feature sets rather than pulling a series circuit and causing the whole ring to malfunction if, say, a single weather generator is destroyed.
- It's entirely possible we could see Covenant stragglers or even sealed-up Flood on Alpha Halo (the ring Locke is on) in this series. A majority of the life there was annihilated when the Autumn blew up, but particularly fanatical Covenant could have gone back when the Halo fragments stabilized in order to look for survivors, relics, etc.
It's an interesting setup, for sure.