Back almost 50 years ago I briefly dated a YL whos father owned a ranch at Dragon Lake near Quesnel. He had some cattle on a grazing lease and we rode out to see that they got home before the weather socked in. I grabbed a pair of chaps from the tackroom. Not much choice, and they seemed quite heavy, my guess close to 12 oz.
The proof that they were not too heavy was when I had to crash through the pine forest after those cows to correct their errant ways. Stub ends of pine branches just broke off on the chap leather without harming me or the chaps.
Generally chaps need not be that heavy. For show, 4/5 oz. is mostly what I've seen. Cowboys with more recent experience should chime in.
Here is a great book to get visual information on many western items, and with many chap variations; Cowboys and Trappings of the Old West;
http://www.amazon.com/Cowboys-Trappings-West-William-Manns/dp/0939549131Bob; Older chaps had the "yoke" almost straight across, while the modern fashion is to have it dipped.