My seedlings are the same, one has taken off and one is still small. Sometimes you just get runts but what you may find is later in its life it may pickup and catchup.
I like to transplant all sorts of plants when their roots hit the edge of the container they're in. I don't like them to be on the edge for too long coz the plant generally knows when its in a confined space, give those roots some more room i rkn. Some people like to hold their plants in containers until they have a bigger root ball though.