Sounds like your Keurig is stuck in the half-cup brew setting.
When you brew with the k-cup inserted, some of that water is used to saturate the grounds in the k-cup, so you would get less "liquid output" with a k-cup inserted than with no k-cup inserted.
Try switching between full cup and half cup setting with no k-cup installed. Is there a noticeable difference in the output from the full and half setting? If not, then it is stuck in half setting.
Look at this post, to help: only brews small cups regardless of setting
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
0