Copied iMac to MacBook Air using Migration assistant, but laptop is using almost 6Gb more SSD space. Why

Recently bought MacBook Air to use while travelling (leisure, not business) and thought the ideal solution was to use Migration Assistant to copy the iMac completely. Both machines work fine, but the laptop uses almost 6Gb more SSD storage than the iMac. Checked storage on both and there are differences in a few sectors, with pauses and minuses on both sides, but the net effect is as described. Any suggestions to overcome this, please or do I just need to live with it?