A military dropship has a very small crew, but are designed to accommodate a great many passengers. The crew living space is largely separate from the passenger accommodations, and is generally more specious (crews spend all their time aboard, so require more creaure comforts)

As a rule of thumb, for each mech/ASF cubicle, there is room for 2 pilots (usually in 2-man rooms, although dropships with room for company+ sized units will have single rooms for officers at the company level and above). For each mech/ASF there is room for 4 techs (usually in 4-man rooms, but for larger dropships senior techs could have double - or more rarely - single rooms).

There is also room for security troops, about a platoon of regular infantry for a Union-sized dropship. Leopards can take a squad, maybe two. Overlords have the space to take a full company. Security troopers will have secondary (and often tertiary) duties as well; they serve as medics, cooks, cleaners, astechs, help with readying mechs, and a million other things. Troopers are bunked 4 a rooms, with senior NCOs/junior officers enjoying double rooms, and officers at the company level and above single rooms.

Finally dropships can be manned with auxiliary staff, such as doctors, dedicated cooks and pursers, anything to make the journey safer and more pleasant. Small ships like the Leopard might have only a few such aux staff, but a Union class will typically have a ship's doctor, a nurse, a chief cook, a morale officer/equivalent, and a small logistics cell (extra labor provided by troopers). Larger dropships, like the Overlords, has room for a larger, semi-permanent support staff.

So a Union-class could have a permanent crew of only 14, room for 24 mechwarriors and 4 ASF pilots, up to 56 techs and astechs, a security platoon of about 36-40 people (who also have many other duties aboard), and usually no more than 10 aux staff. If fully loaded, a Union could carry as many as 140 people, plus 12 mechs, 2 ASF, ammo and other combat consumables, spare parts and replacement armor plating, general supplies for long trips, some light vehicles, and whatever else can be crammed inside (if you didn't bring it, chances are you must do without for the duration). By comparison the much smaller Leopard is hard pressed to accommodate 60 people, while the massive Overlord can carry half a thousand men between the stars.

Typically a Union won't carry as many as 28 pilots, which frees up some of the more desirable accommodations. In this day and age it's also rare to have enough qualified techs to fill all the berths - some merc companies make do with less than half as many (but that largely precludes any heavy maintenance while en route). It's also common to skimp on security, because who actually tries to attack grounded dropships? A typical Union might have about a 100 people on board, maybe a little more (120 is usually the "ceiling" most commanders won't exceed). Less crew means lower operating costs and more mileage out of supplies - and a much more pleasant travel experience. It's really exhausting to spend weeks, months even, aboard a fully loaded dropship.