I think there's more pros to that than cons ...
I was used small sandia machine which when u look into the clean tank ... you see all there is to the capacity.
But getting a miniflex then i poured in a larger bucket of water ... looked in ... where'd it go !
two more big buckets and still space .
So reach you hand into the empty tank ... there is not only pockets of space going down the front , but also right towards the back under the waste tank .
The front pocket is good in that the pump water siphon ( which you can remove ) is stuffed in there and is always well below low lever as well as being held in place firmly.
I would usually leave a couple of liters in there , i dont back mount the machine as its so light i just lift it in upright .
But if i plan to take it upstairs for eg , i stick in the vacuum hose and suck the water pockets out to save the extra bit of weight .