A creature with LargeStature is larger than the imperial norm. Common examples include the Ogryn.
This causes all kinds of problems - from low ceiling heights to small furniture, beds and clothes. Large creatures also consume a lot more food, water, and air than normal people.
The advantage lies in their ability to use larger DragonGuns. Someone with LargeStature uses all guns as if they had one concealability class less - so pistols for them normally have a concealaibily of Negligible. N/A becomes Trenchcoat and Support becomes N/A - allowing the use of support weapons as normal carried weapons.
However, weapons that in themselves have a concealability of Negligible are too small for someone with LargeStature to use.
This does not apply to melee weapons - which are scaled to accomodate the user. However, someone with LargeStature can never use the Close melee weapon modification against someone smaller than themselves.