It could be that they specified 7 inch joists to allow for pipe
cut-outs etc.

Most properties I have seen have had notches cut out way over the size
of the pipe so I would not worry too much. If you are still concerned
maybe bolt a small piece of joist with a joist hanger along side.

I had a structural engineer visit recently advising on me removing a
central brick wall alongside the stairs. My ideas to reinforce were
way beyond his suggestions. He seemed happy to hang a couple of joists
on a single brick wall only 18 inches wide and 16 ft high!