My answer to your direct question is much as posted above by others. A 38L pack is rather small for even an overnighter unless you have lightweight gear and it is easier and cheaper to change the pack than to replace all the gear.
I currently use a 44L Osprey pack. I can just about squeeze in everything I need (including food and three 500ml water bottles) for a night on the hill. But I could make things easier for myself if I chose to. One of the reasons it's all a bit of a squeeze is that I like my comforts. So I take a SIM and also a compressible foam pillow (which, though light, adds bulk) whereas if I was less fussed about my comfy bed I could save weight and a fair bit of space by substituting a rollmat (CCF mat) and an inflatable pillow.
Re your sleeping bag: do you fully compress the syn bag? Have you cinched the compression straps tight as they'll go? That might save a few centimetres perhaps.
Re your tent suggestion: I've got a Zephyros 2 and it's not particularly light (can't remember off-hand - just under 2KG I think). As supplied, it's not particularly compact either. If you leave the two end struts in place with the tent rolled round them the pack length is about 400mm - 450mm (I can't measure it just now because it's in the loft). But there are suggestions and ideas on re-packing (including modding the end struts) on the OM Forums Zephyros Owners thread.
Re carrying items on the outside of a pack, opinion seems polarised - some people seem to regard it as a mortal sin. To me, it depends to a degree on where one is walking. For example, in undergrowrth and woodland, external items might be susceptible to snagging and/or ripping. Or on difficult or steep going, your balance and centre of gravity could be affected. But for general hillwalking, I personally don't mind carrying stuff on the outside of my pack.
Edited: 04/08/2012 at 19:32