Barbey Dustin says that she contributed as few of her men to the Stark host as possible. Would the Starks not be aware of how many men she can raise, and questioned this disloyalty?

Well, this gets us to the twisty nature of the feudal contract. As bilaterally-negotiated documents, feudal contracts could vary dramatically in terms of what kind of service was negotiated –  how many knight’s fees your land was valued at, how many days’ service you were required to provide, how many men you had to bring, etc. 

What this could often result is that there was a difference between the minimum a lord was required to kick in and the maximum they could actually bring to the table, and how many men actually showed up would depend on politics. If the king is popular and/or powerful, if the war is going well and there’s a good chance of winning loot/land, you bring extra men above your minimum requirement so as to gain royal favor. If the king is unpopular and/or weak, if the war is going badly and the risk/reward on participation is bad, you send as few as you can get away with. 

We see this very early on in the War of Five Kings with Bran VI of AGOT: the lords who show up in full force to Winterfell are looking to gain something in return, whether it’s a military command, or Robb’s hand in marriage, or for him to give them some land or some use-rights, or to side with them in a dispute or what. Barbrey Dustin is making much the same political calculation, but in reverse: what’s the least amount of men she can get away with sending without incurring a felony?