When i try to brew a large cup it has started brewing small cups instead. I have to brew two large cups to get enough coffee.

I have one of these coffee makers also and having the same issue. The machine is getting limed up. Run some vinegar through it. What I found also a little trick is to turn the machine on, place a cup underneath like you were brewing a cup and press the brew button. Let the remaining water come through and the next time you will get a full cup. But this does not solve the issue. Liming seems to be the issue with these makers.

If you don't brew often enough you will loose a small amount of water in the tank due to evaporation while the tank heater comes on and goes off to maintain the proper brewing temperature, so if it has been a while since you last brewed, without coffee in brew basket put a carafe of water thru the brewer, you will get less than a full carafe out because it fills the evaporated space in the tank first then uses the rest to brew. Once your tank heater shuts off usually 5 minutes on less you will get out all the volume you put in.

This problem is addressed on the Keurig website:

http://www.keurigtroubleshooting.com/Keurig-Coffee-Brewer-Problems.html

Scroll down to Partial Cup Brewing

Hi,

Here is a tip that I wrote that will give you some things to try that will help you get your Keurig coffee brewer working again.

Keurig Coffee Brewer not making a full cup of Coffee

heatman101

The coffee maker has three contributory factors for brewing experiences. First, putting enough water in container prior to brewing is expected to dispense in to coffee capture cup. Second, installing the component parts after inserting coffee leaflets should not hinder brew seepage into the cup. Third, allowing enough time for the brew to dispense is a requisite means to coffee impressions. Furth, coffee leaflets require appropiate numbers inserted at the container area lest the machine takes time to sock and dispense. However, refer to the device user guide for appropiate manufacturers instructures to user as expectations during brewing. Also, refer to the user guide to ascertain any conditions that may be signs of damage preventing fast brewing.

I have have the same problem first thing in the morning. My first cup, I press large but with no K-cup; it comes out small or medium. I put the water aside (pour it back into the reservoir after it cools.) Then I brew a large cup using a K-cup, and it comes out the right size. I still want to know how to fix the problem, but at least this works for now.

How does the coffee get into the cup? If it brews directly into the cup then the amount can be controlled by how much water you put into the maker. If it is a 6 cup then it must hold the coffee in a reservoir. Do you have to push and hold a button to dispense the coffee or do you push and wait for it to fill your cup? If it is the latter then it will need to know how large or small the cup size is. Is there a setting for that? If it is a push and hold method then you control the coffee flow so size is up to you. As far as it being a travel mug or ceramic mug as long as the mug will fit in the maker and sit securely under the nozzle then it shouldn't matter what kind of mug it is.

Finally, I got my coffee pot to run through without stopping several times. To fix it I had to run vinegar through eight times! Never before has it taken so many cycles to clean out a pot. It is still spluttering a bit so today I am going to get more vinegar or maybe even CLR and run it through a few more times.

Yep, it's the one inherent flaw in the single serve machines - someone else decides what 'the perfect cup of coffee' tastes like.  Quite presumptuous, actually.  
Solution I found to be acceptable:  Buy only the extra bold flavors (there's about 5 or 6 varieties) and brew using the small cup size button.  Yes, you only fill your mug about 3/4 of the way, but at least you get a decent cup of coffee.  
The ONLY flavor I found to be strong enough when brewing with the large cup control button is DOUBLE BLACK DIAMOND...the rest is crap.