There are two base assumptions:
- Lannister Army has reached their destination and is operating in their deployment area.
- Lannister Army still hasn't reached their designated area and is en march.
If we assume that Lannister have reached their destination, small parties should not be surprising.
Their mission is maintain the peace and stop the political situation from devolving into general mayhem. The said mayhem is caused by resistance fighters, bandits and Broken men.
These offenders are generally found in smaller groups and retreat when they see a larger Army coming their way. Not to mention, Larger armies are easier to track than smaller parties.
That's why small patrols have a greater chance of success than moving the entire army all the time. It is also cheaper as small parties can easily fend for themselves whereas armies require upkeep.
Also, there's a chance they might be foragers but foraging is mostly done in wartime when resources are scarce. Also they don't have carts to haul their booty so I doubt they are foragers.
If the Lannister Army is still on their way to their deployment area, it is customary to surround a marching army with scouts and outriders to get an advanced warning of an ambush. No smart general moves without spreading his eyes and ears far and wide.
It is possible that Arya actually met scouts rather than the main body. The men are too few to be rear-guard or vanguard of the Army so most likely they are a scout unit.
Sure there is risk of desertion in these scenarios but that risk is present all the time. And Lannister Army may be tired but they have won the war. So their morale can't possibly be that bad. Conventional wisdom dictates that their commanders should send out small parties out both in march and when they reach their area of responsibility. The possibility of notorious outlaws such as tBwoB is very strong as well but that's their mission isn't it? Find out outlaws, root them out, if outnumbered, return back to the base and report your findings.
Case of Polliver's men
It is a saying that there's a beast in every man which comes out when you thrust a sword in its hands. That particular band is sworn to notorious Ser Gregor Clegane. They seem to be doing what some soldiers like to do i.e. rape and plunder. There's more for you to rape and more for you to plunder if your competitors aka comrades are less. Who's going to oppose them anyways for them to move in large parties? All their enemies are defeated. They could go out on any pretext such as foraging or scouting and fill their pockets. The Local lords are unlikely to resist to King's men.