I logged in after a few hours and the bag is no longer reducing my stamina by more than would be expected, however, after putting it back on and relogging it went from "about 1kg" to "about 6kg" when I logged back in. I put some stuff in it and then removed them and when I scrolled over the weight again, it had gone back to "about 1kg". When it stated it was "about 6kg" it didn't seem to change the actual stamina bar by more than what you'd expect from a 1kg item though, so it seems to be an information issue rather than a actual weight/stamina issue this time.