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Your machine should have two buttons for the choice of small or large coffees. If youve been trying these and theres no appreciable difference in the volume produced perhaps theres a fault.
The best way to heat cups is to fill them with hot water from the machine. Do that a few seconds before youre ready to brew, then empty out the water ,and brew. The cup-tray warmer is not hot enough.
Have you tried taking the temperature of the coffee in the cup with a thermometer? Above 85C should be ok
Let us know how you get on.