Hi, I am fairly new to serious backpacking. I don't consider myself ultralite, but want to become at least lightweight. Right now I have a 50L pack (Osprey Atmos, older model). I love the pack fit and it is pretty light, but I currently have an older synthetic 20* bag and it doesn't pack very small (i.e., with the compression sack it can hardly get in my pack. Any recommendations for a 3 season bag on a budget? Packing size is more important than ounces. Right now I am packing the old bag without compression and it takes up way too much space.
I'll be hiking solo and for 2-3 days max.
I suppose the other option is to just get a larger pack, but I feel like that shouldn't be necessary with such short hikes.

We still use our old REI sub-kilo bags and love them. They pack up to the size of a large loaf of bread, weight about 2 pounds, and are rated to 20 degrees or so.

WE got them years ago on a close out for under $200. You can get something similar for under $120 if you go for sythentic...although they will weigh about a pound more

I think we automatically assume that bag size increases or decreases with duration of the trip - that isn't necessarily so. The volume of your pack should match the gear, not the duration of the trip. Gear, if you get it dialed in and it's adequate to 3 season conditions where you're going most of the time, doesn't change. What changes is food - if you need more or less food, or if you need a bear canister to store it in.

Although, if you are changing up and playing sherpa for a girlfriend or kids, that too can be a game changer.

For me, quilts fill the bill without filling the pack, and my go-to pack for trips up to a week is a 40 liter - bumps up to a 50 liter if i need a larger bear can. One of my quilts is good below freezing, with the right sleeping pad. That's true of any bag as well since insulation on the bottom of a bag compresses.

I like my Kelty Light Year Down 20º bag. Its comfort rating is 32º I think, and it keeps me plenty warm at that temp. I use a down jacket over it down to 20º and that keeps me warm.

I paid about $130, but I've heard of them going for less now.

Since size is of primary concern I'd definitely look into a quilt. Depending on how cold your shoulder seasons are and how you sleep you can find some good deals on 30-40 degree down quilts that might be exactly what you need. These compress really well and don't have the wasted materials of a bag.

I have a Kestrel which is probably similar to your Atmos in terms of bag compartment space. I am able to get my quilt, a pad cover and the body of an old NF Tephra 22 I use to keep the bugs off at night into that area.

Oh and don't let your backpacking become too serious. It is supposed to be fun!!

If the bag is warm enough, I'd try something else. So not use the compression bag. Put the sleeping bag in a large plastic bag and leave the top open so you can squeeze the air out as you pack things on top of it. Make sure the top is folded over so water cannot get in. It should give you some extra room.

Those go hand in hand actually because no synthetic will pack as small as a down bag, which would also be lighter. In the past few years, I've seen some entry level down bags (eg, Kelty Light Year) as low as $80. I have a REI Sub Kilo and saw those on clearance for about $130 within the past year. The other option is buying used. Backpackinglight.com has a very active gear swap forum.

I'd also recommend a quilt (I've used my bag as such 99% of the time), but those are much harder to find. Golite's will be the easiest and cheapest to find unless you go with a used one. Synthetic may also be an option again if using a quilt. If you're handy at sewing, you can make your own for maybe $70.

Hard to find? Aw, bull.

jacksrbetter.com
hammockgear.com
http://www.warbonnetoutdoors.com/topquilt.php
http://www.enlightenedequipment.com/

That's just for starters.

Google Ray Way quilts, or go to thru-hiker.com, for quilt kits in synthetic or down.

A good quilt must be paired with a good pad. Also a good hat. Quilt sleeping was something I adapted to very quickly, but others have told me they couldn't get it tucked in. If you are a big guy, look at Jacks R Better first - they have larger versions of all their quilts.

Hard to find as in you won't find them in any stores aside from the few Golite locations. Plus those options are more expensive. And I did recommend the MYOG path.

Mountain Hardwear Ultralamina 32 degree bag packs down really small! Of course if you need a full zipper, do not go this route. I have the same bag in 15 degree and it's my winter bag for this area. I need another bag (and I'm looking at a quilt) for the rest of the seasons. By the way, I'm assuming by your name you backpack in North Carolina? I'm near the NC boarder in Tennessee.

Edit: To go back to what Lori said, Enlighten added straps to his quilts where you can tuck the quilt around yourself and the pad. Really, really cool!