Marines, lose 1v1s, most of the time. By design.
With a few exceptions, if a marine is winning 1v1 vs a xeno, it's a skill issue. A runner vs a marine, a drone vs marine, a sent vs a marine, all of these are intended to end in the xenomorph's favor.
Mechanics such as Pounce, Tackling, Marines taking fractures and bleeding, all of these are meant to incentivize marines to stick together even more than xenos are, since, a pounce on a lone marine is a death sentence, but, you can only pounce one marine at a time!
And this doesn't just apply to T1s and T2s, either. A crusher is designed to take fire from dozens of marines at once, an opressor's hook is not meant to literally decimate a squad when it hits, a queen's screech is meant to stun a screenfull of marines- with the expectation that there's two more screenfulls ready to pounce if she does it wrong.
This is the inherrent design philosophy here. Marines are weaker than xenos, but, they have the numbers to make up for it.
But! It goes further than just in combat; marines are significantly more reliant on eachother than xenos are.
Where a lurker on their own can run off to weeds to heal any damage they receive, a marine needs medicine. Trying to splint your own fracture takes far too long to be practical, you are entirely unable to fix organ damage on your own, if you run out of ammo you need Requisitions to send you some down or else you have to go all the way back up to the ship to get some.
And on that note, how are you getting back to the ship? You need someone to pilot the ship!
A marine who dies on the frontline, gets revived, drug back to the FOB, and sent up for surgery has only done this by the grace of at least 3 other people now.
In order to bust a chokehold as a marine, you rely on air support, CAS, OBs, Mortar. All of these require the communication and teamwork of at least two people, all of them needing specific people, at that note. Without a PO, there is no CAS. Without an MT, there is no OB. Without someone in CIC, there is no OB.
So, what happens when the number of people in this complex web of inter-connectivity is cut to fourths? Well, it all falls apart, obviously.
Marines on Lowpop are almost always bereft of at least one of the major factors that is thought of in their balance.
Whoever is aCO is oftentimes the ONLY person in both CIC, and their department, forcing them to balance the two in an act that's frantic to keep up with, if they bother to try at all- and certain aCO roles can't even do the whole CIC job anyway.
RO, CMO, SOs, none of them can load the OB.
CE and RO can't wear a medihud to give better overwatch.
Only the SO and XO get dropship access by default, everyone else needs to take even more time out of their incredibly busy job to give themselves access.
So, here we have a force that can be reasonably called a 'handfull', with about 6 more marines than there are xenos. They have no firesupport, they have no supply line.
They do their best. But Xenos are a lot harder to kill than marines. And every cap is -1 marine and +2 xenos. It only gets worse for them at drop.
And remember how I said Marines are supposed to lose 1v1s?
It's a whole lot easier to find a lone marine when there's 23 marines total.
I've seen 5 Xenos turn around battles from hivewipe against 40+ marines.
What do you think 10 is going to do against 20?