That's not that simple.

Let us imagine the following configuration.

We have 3 servers each of 4 slots, below is the state of each server (the state of a server is a set of VMs it's running)

[3], [2, 1] and [2].

Say we need to place a new 3-slot VM to our system. We'll pick the third server as less-loaded and try to accomodate the second server. Together they have enough space, but we can't provision the VM withour using all 3 servers. The solution here is

1) migrate a 1-slot VM from server 2 to server 1

2) migrate a 2-slot VM from server 2 to server 3

3) place the new VM to server 2