Just to follow up, my friends do not go in for checkups very often. They said they just monitor symptoms and have their thyroid checked because a lesion on the pituitary can cause thyroid problems. Every 5 years or so they do an MRI to see if there has been a change in size, but so far after a couple decades they have seen no change. The pituitary gland controls a lot of the endocrine system, so the doctor is probably just making sure your hormones are all in balance. I have a thyroid problem with no lesion, and it's no big deal to get help for this.
And 7mm on a pituitary lesion is tiny. My friend who gets super headaches said hers is 1 inch, which is about 25 mm. So that is probably why the doctors aren't worried at all.
Probably what they are doing is getting baseline measurements. If the doctor is worried he would probably say something to you. If he isn't worried, this is probably routine tests just to make sure the lesion is not effecting function of the pituitary gland.
I would ask the doctor point-blank if he is concerned and see what he says. It's not fair to make you wonder. Part of his job is to support you through the medical process, and explaining why the tests are done is only fair.