1.
to keep increasing in quantity until there is a large pile, a long line, etc. :
Cars quickly stacked up behind the bus.
2.
stack (against sb/sth) (used especially in questions or in negatives) to compare with sb/sth else; to be as good as sb/sth else
SYN measure up :
Let's try him in the job and see how he stacks up.
A mobile home simply doesn't stack up against a traditional house.
3.
(used especially in negatives) to seem reasonable; to make sense :
That can't be right. It just doesn't stack up.