That im affraid is the answer , i could be wrong though , i think its to do with friction of water in the smaller bore of hose .
I use 12mm at the mo and am looking at going to 6mm , im only on a 60psi pump , if it cant handle the smaller bore , then 100psi it is then !
Look at it this way , you could always use your old pump to set yourself up with a DIY trolley , thats my plan .
Rich P @ F