Big enough for what?
I find the 68L of Vario's adequate for the two of us, but would prefer a bit more capacity (the 80 odd litres of a GSA sounds about perfect) but for solo use I could easily get away with the vario's in compressed format which is probably about the same as a couple of 21's.
42L to myself would be more than enough, currently I have the small pannier and she has the big one, misc crap goes in the top box (spare gloves / visor / jumper / ear plugs etc)
I also use Hotels, if camping I would probably need more space, but I guess of your solo this can all be bunged on the back of the bike.
If this is for the odd little trip to carry one persons gear and stopping in Hotels I guess it should be OK - and the price sounds pretty decent.