Space would be a key element. Most people only experience the "tender" side of the operation, when capacity is much less than it will be if an evacuation (100 for tender, 150 for lifeboat, typically, for HAL's lifeboat/tenders), and don't know what it will really be like in lifeboat operations.
There are a couple of YouTube videos explaining the real situation in lifeboat operations. But to your point, I am reasonably sure the lifeboats are designed with some excess weight capacity. Physical space requirements are a different issue.