using network monitoring software, I dont know how it can be bigger though as packets are all individual from host to host, not added together, is this right ?

If your uplink port is a trunk, you might be seeing a slight increase because of the tags.

If the uplink port is a routed link, the size might be skewed by blocking some broadcasts and/or multicast. Also if you're using a IP helper, recall the original packet might be repackaged.

If the uplink has a smaller MTU, fragmentation would increase the total bits being transmitted. Individual sizes should be smaller but an average packet size could increased.

"not added together, is this right " Correct.