I read this book back in 2005-2009, but it's possible that it might have been released earlier.
The key points I recall are that the protagonist worked as something like a border guard, or at least lived on a border between his country and a magic forest; the forest's trees absorbed dying or dead people into their collective consciousness; and the mages of the forest were both fat from magic, and went around placing the injured at the trees.
The protagonist was against the absorbing, believing it was bad. He grew fat from using the magic of the forest (by accident?) and didn't know what to do. At one point after growing overweight, he had to return home to his father's estate. His father was horrified to see his son had gotten so fat. His father then proceeded to lock the protagonist in his room, refusing to feed him anything but scraps, then eventually nothing, believing his son was stealing food since his appearance didn't change at all.
The protagonist tried to keep his mind off his situation, instead feeling the warmth of the sun. He eventually escaped his father's estate and returned to his border house. He ends up having to bury some bodies, but then is horrified to find that the forest is eating them. He tries to untangle the bodies but they plead for him to leave them be.
At one point, he ends up in the forest himself and winds up at a feast hosted by the mages, and finds himself thinking that they're beautiful. The mages introduce him to why they let the forest absorb people, and he finds himself linked to the spirits absorbed by the forest, happy and cheerful. That's the most I remember.