When RC papers came out, they were labeled "medium weight". Fiber papers generally came in single and double weight and the RC was somewhere in the middle thickness-wise. Today's fiber papers are essentially the old double weight thickness. I haven't seen single weight paper in decades. Others may know of some.
If you think RC paper is thin, you'd hate the old single weight papers. Find some old prints from up through the 1960s - even old family snapshots. That's probably single weight paper.
I like Portfolio a lot, but for contact sheets I want something thinner to save space in the neg binders. I'd use single weight if it still existed. Last time I saw it was POP paper and Azo. Single weight was great for paper negatives.