I'm facing a big trip next week (16 days and 4000 miles) and, although there's 2 panniers on the bike and I'm taking the top case (Givi E460) I realize that I can only use one pannier for the luggage. The reason is that, if I want to visit a city (I checked out from the last hotel but still didn't check in in the next one), I'll drive there and then leave all the riding gear in the different luggage compartments on the bike in order to be able to do sightseeing. The free pannier will be taken by the helmet, and in the top case I can barely fit the boots, the trousers and the raining gear. That leaves me with the riding jacket, which is not so bad when walking around a city, except if it is too warm.
How do you guys deal with this?
Any ideas to improve the storage? Since my topcase has 46 lt capacity, I feel kind of stupid shelling out a ton of money to get a Givi Maxia E55 (55 lt). Am I being cheap? 

Just because you haven't checked into your hotel yet doesn't mean that you won't be able to store your gear there for a couple hours.  I took a train to Brussels, Munchen, Koln, each of the hotels I stayed at in those three cities allowed me to drop my bags there since I had arrived several hours before check in.  I would call your hotels, the worst they can say is no.

You could lock the helmet to the bike outside the pannier. See:
http://web.ncf.ca/ag136/helmetlock.htm

You could lock the helmet to the bike outside the pannier. See:
http://web.ncf.ca/ag136/helmetlock.htm

will that work on a C-14?

Just because you haven't checked into your hotel yet doesn't mean that you won't be able to store your gear there for a couple hours.  I took a train to Brussels, Munchen, Koln, each of the hotels I stayed at in those three cities allowed me to drop my bags there since I had arrived several hours before check in.  I would call your hotels, the worst they can say is no.

+1  I've rode into town before, dropped off stuff before check-in... most hotels don't have an issue with this at all.

I've seen guys buy a cable lock and run it through the sleeves of the jacket

I always carry a bike lock and I can lock up to four helmets to the bike.

I use a bicycle "seat lease cable lock" (goggle "rei cable locks for bicycles") that I run through the fastening rings of the helmet's chin strap and let the helmet hang off the rear rack of the motorcycle. The cables are used on bicycles with quick release seats to make it difficult to steal the seats.

OK......
I guess I must be the only one who does this.
I shed my gear, thread a cable lock through the pant legs, jacket arms and helmet strap, and thread through my Heli-bars, all on the seat.
I then cover the bike with my full-size bike cover, you know, out of sight out of mind.
I have even used a personal alarm that is attached to the hooks on the covers bottomm if it is pulled on from either side, it lets out a 130db screem for 2 minutes then shuts off. But I have only used it a couple times, and really didn't seem to need it.
I have never had a problem with this on any bike, from my early on 650's , my Gold Wings and now a couple Connies. A good full-coverage cover and a cable lock does just fine.
I also know, that if they want it that bad, they are not going to mess with the individual crap, they are just going to take the whole bike.
Good luck with what ever you choose.
Ride safe all.
Greg

I use a 6 foot long 1/4" thick (or maybe its 3/16"? - been a while) coated cable.

Bought it by the foot off the spool at the local hardware.

Put a loop in each end using the press fit ferrel's.  I think it cost about $5 total.

Thread the cable through helmets, coats, pants, whatever.

Secure the loops (or one loop if you thread one end through the other loop) in the helmet lock (C10 reference here) - eliminates needing an extra key or fiddling with a combination.

Pfloydgad's method with the cover on the bike is a even better.

A self-coiling Wal-mart bike lock is about $6.  Check the combination plate and you can pick a combo you like (or can remember easily).  Some can even be reprogrammed.  I carry one in the trunk all the time so I can lock up mine and my wife's jackets to a grab handle.  I have also relocated my helmet locks to the liscense plate bracket so the helmets can be locked there.  Rain would be the only concern in this set-up (maybe vandalism on a rare occasion, but most people won't mess with a "biker").

guys, thanks a lot! That's a bunch of good advice/ideas. I'll see what I can implement.
@Jeremy, you're right, most hotels will let you leave your stuff before check in. I was asking about those cities on my route that I'd like to visit but I won't be staying. Sorry if I wasn't clear. 

The cable lock works great through the jacket, etc.  One thing I found last year was the eyesight is getting older and my reading glasses were in my bag.  It was a bit tough to read the combo as it got darker and I was without the glasses.  I switched all my cable locks to keys.  I know the down side is I could lose a key.  I have them all keyed alike and I generally don't lose keys. 

I also mounted a helmet lock onto my MC Ent. tip over bars.  A helmet takes a lot of room in those bags if you put it in there. 

In a pinch, throw in an empty backpack in your luggage, then just pack the stuff that won't fit in the bags, and carry it with you..