I have three different worm bins. Two are kept outside. One of them is for dog poop only and the other is for general food scraps, leaves, cardboard strips and a bit of newspaper.
The inside bin is also for food scraps and such, the same as the general bin outside. When you say that they worms seem bigger and fatter, that could be a couple of things. One, you could be looking at the same worms as they get older and more mature. Obviously they start off small and get bigger. One thing to look at would be if they have a clitellum, the ring around one end that contains the sex organs. If a worm doesn't have one of these it is just a juvenile and not fully grown. It seems better to have all your worms big and fat, but think about it, if they are not breeding and giving you the little skinny worms then when they reach the end of their life cycle you will be left with nothing. Lots of small worms is a sign that the conditions in your worm bin are conducive for breeding.
Last winter was my first real winter with worms and I worried that the ones outside would freeze. I live in Flagstaff AZ at 7000ft so it is definitely a possibility. I caught as many worms as I could and transferred them to my inside worm factory. I even tried a small heating pad under the worm factory to see if that would make things better. However, I let the wormbin build up too much moisture and a lot of my worms died.
In Spring I went out to see if there were any worms left alive in my outside bin. I pulled off the compost-bin and had a frozen mass in the shape of the bin. then I hacked at it with a pick-axe until I broke it into chunks. I found that down at the botton, in the middle, was a section that wasn't frozen at all, and in this section was all the worms in a big ball. I don't know whether their body heat or possible secretions kept the section from freezing or it was just luck that it wasn't quite cold enough.

Either way, I took these worms and put them into my inside bin and they have been doing very well. Even having taken all the worms I could see from the outside bin, the outside bin was soon full of worms when the weather warmed up, probably from cuckoons that were laid before winter got too hard. I have heard of a technique of "stressing" the worms deliberately by letting things dry out or otherwise fool them into thinking that they are all going to die so that they are fooled into producing many cuckoons as insurance against the stress, so that their offspring, lying dormant until conditions improve, would still survive. Presumably, before all the adult worms died, you would reverse the stress conditions with a cry of "just kidding!" and then benefit from the resultant baby-boom regardless of the psychological havoc you have wreaked on their poor parents.

"stressing" I'm going to try that even if I have a healthy population. . . a couple weeks ago I must have overfed (a few weeks prior to that I moved worms to 10 gallon container) them and the bin was a little stinky (tart smell) so I mixed in some SHREDDED newsprint last weekend and again today. I did check and worms are in there, no suicides - plus it is muggy & rainy. Cover is off. Hope that helps "dry" out the bin a little and no odor like before AND stress them just a little so I have more!

Thanks wormlover1