The reason it is suggested to hold of solids until 6 months as a lo's tummy is very sensitive, and can end up causing problems such as constipation, allergies etc. Baby rice can be very constipating for a lo.
I have taken this information from the FAQ's section of the Solid Foods Board:-
Other points to consider: Younger babies may have difficulty signaling to you when they are full, and their digestive systems may not be mature enough to handle solid food. Gas, tummyaches, and constipation can happen anytime during solids introduction, but consistent digestive problems may be a sign that your baby's body isn't ready yet. Also, there is no conclusive evidence that solid foods will help your baby sleep through the night.
Some bottles in the UK go up to 9 or 10oz. I have heard of people adding extra to another bottle if their lo is still hungry. All babies are different and take different amounts of milk.
HTH's
Paula