Sounds normal to me. When I was first reading up about BIAB, it was mentioned in several places that this is an issue. Absorbing 3/4 of 5 gallons seems like a bit much, but it will absorb some (I've heard about a quart per pound?). It probably didn't absorb that much, as the wort would drain out of the bag if you could hold it over the pot for 10-20 minutes.
I've adapted my technique for BIAB over a few batches. These steps are my version of merging BIAB with all-grain and sparging, and helps with this issue. I've used this technique with all-grain recipies, and associated water temperatures for mash and sparge, and it turns out great.
Don't start with all the water in the pot. Then if you need, you can take the pot down on the floor to get over the bag to lift it.
Put down towels when removing the bag. It drips... a lot.
I got some buckets from WalMart, drilled holes in the bottom of one, and put the grain bag in the drilled bucket, and then that bucket into a second bucket. This will let it drain without having to hold it up the whole time. If you can get the pot on the floor, then just stand over it with the drain bucket right next to it. Then just lift, pivot, and lower the bag to start draining. Have an extra catch bucket ready for when the first one starts getting full. (Also helpful for the next step.)
After not having used the full boil volume in the pot, we need more water. I heat up another few gallons, and begin pouring it over the grain bag in the fermenter bucket. The fermenter bucket is big enough to hold the extra volume you'll be pouring in there. This serves to rinse more of the sugars out of the grain, while topping off the volume. (This is my version of sparging.)