When looking at flow - volume the hose ID will have an impact on these as already commented the hose wall itself does create a restriction as will the expansion rate of the hose. When air temps are cooler the hose will not expand as much or as quickly. A hot system will mean the hose wall is a little more supple and mean the pump does not have to work quite so hard to move the water as in Daz comments. This also allows the flow rate to be turned down with out effecting the anount of water delivered to the brush
Also a Hose has a maximum capacity of water it can carry with 8mm I have tested this is about 2.7lpm. Once this maximum capacity is reached pushing the pump harder will not increase the LPM at the brush head, It just means the pump is working hard for no benefit drawing more current than it needs to and getting hot.
6mm will have a lower max capacity than 8mm. In technical terms we want a smooth flow of water through the hose with the least resistance, (Laminar) This is the water all moving in the same direction at the same rate. Bit like a nice free flowing motorway. Once the Hose capacity is exceeded the flow begins to move at different rates and directions within the hose (Turbulent) Now the all the three lanes of traffic is trying to get through using one lane and everything slows down and becomes start/stop rather than free flowing.
This blog gives more detail
http://springltd.co/blog/84/flow-resistance-and-volume