crammed in like sardines

Crammed in like sardines is a figurative way of saying a large number of people are placed in a very small space in comparison to how many people are there. Literally speaking, sardines in a can are so tightly packed, they are often hard to remove; there are dozens of sardines in one tiny can. As in the figurative sense, saying you are crammed in like sardines means so many people are in one space you can hardly move.