I as looking at a 45 liter pack and then saw a version that was 65 liters on sale for nearly the same price.  I know that if I get the bigger one, I'll be tempted to fill it.  Are there issues with under loading a bigger pack?  Maybe extra sloshing around if it isn't cinched tight enough?

Don't get one bigger than you can carry

I'd be fine with the 65.  I'd rather they have a 55 though.  I'll probably end up getting the 45.

If you have cinch straps on the pack it should be fine. I often under load my 50L pack. I cinch the bottom strap tight and it keeps the load quite high where I like it.

Get both.  Or rather, have many - one for every circumstance.

Bigger packs tend to weigh more. You have a tendency to over pack a bigger pack.

It really all depends on what your using it for. I use a 65L got multi-day trips. 

My use would basically be for an overnight pack but have it ready for the possibility of being stuck for 2-3 days.

50L range will cover this as long as you don't run a big sleeping bag.

I would split the difference and get the size in the middle, then its good for short trips, or multi day.  -Of course I am not a light camper, I actually bring stuff  

Whichever size I am using if I have enough room left over I like brining a small pillow.  I have trouble sleeping without a pillow, or extra cloths stuffed in a pillow case acting like a pillow.  

**UPDATE**

Bought a Mystery Ranch Ravine pack today.  It's 50L and should do me right.  Now to fill it.

I settled on a two BOB approach after building out a 7500cu. In, Kifaru EMR. My plans included me needing an "I'll be gone for a while" bag more than a BOB, so I lived with a bag that had more stuff in it than I needed for a three day bugout until I could make something up in a smaller bag that I could be light and fast with.

What I found is that once the basics are covered, most of the difference in volume is taken up by food and extra clothes so you really have to choose wisely if you live in a cold climate winter state,

My experience with larger packs is I can't resist the urge to fill them.  

I've had some truly enormous packs over the years (e.g. Tactical Tailor's MALICE V3). At one point, my pack weighed in at 67 pounds.  While I could probably make this work, in isolation, it doesn't work when I add a rifle and ammo LBE. Now I try to keep my pack to around 40 pounds.  My current set up is 43.8 pounds in a Kelty AMRON Map3500.  This is about as high as I can comfortably carry (if I'm carrying an AR with ammo).

Having said all of that ... my pack selection was based upon my objectives.

If your area or plan dictates that you need to survive for a long period of time in an inhospitable environment you may need a larger pack.  Before purchasing a pack I would ....

1) Write out my plan of action;
2) Write out the BARE ESSENTIAL gear I need to suceed at my plan of action;
3) Review and revise;
4) Buy my gear to see who big it is;
5) Delete or replace anything that seems too big / heavy / whatever;
6) Buy a pack to fit the above.

If you don't go through the six steps above you'll end up with a pack that is too small or worse ... one that is too large.  If you have extra space you will fill it ... and will end up barely man portable.

I've been there, and it was embarrassing when I discovered the error of my ways.

As somebody who bought a 45L pack but really needed a 65L...

1. You can usually under-fill a pack and then compress the bag with straps.

2. Where is this on-sale pack you're looking at?

I was looking at the Cabelas Endicott pack.  They had the 45L on sale for around $85 and the 65L for like $100.  I ended up paying full retail for my mystery ranch but I'm more than happy with the build quality.