Well, look at the "payload" of each bike.
IF he is a large guy as you say, and you wish to ride as well (you did not indicate your weight), you want to ensure that you don't exceed the payload of the bike (assuming you'll eventually want to load it down with extra cases, etc). I hate to make assumptions, so if you are in the 100-150 lb range, it would probably be OK for a 400...if you are 150 - 200, that's starting to get problematic, and if you are 200+ you'd better read the specs on the bike to ensure you don't exceed them. Extra weight on a bike will cause the suspension to ride lower, and could cause tire problems in the long run. NO doubt there are 400 owners here who push the limits of their bikes all the time, as do 650 owners.
Essentially, buy what you can afford - get the best bang for your buck. With the 650 you'll have a larger engine and larger payload capability, and a few more HP, but compare them and both of you sit on each one and decide which one fits, feels, and is economical for YOU. If you know somebody who has one, ride as a passenger on a 400 and 650 and see which one feels better (seat, wind, etc).
Good Luck!