What a load off Abby Lee Miller -- she's reportedly getting out of federal prison next month, and she'll be way lighter when she does it.

Abby posted a pic of herself and some friends -- looks like they were visiting her at the federal pen in Victorville, CA -- and her weight loss is obvious in the shot. There are reports she's lost about 100 pounds since entering prison in July.

You'll recall, she'd had gastric bypass surgery in April.

Abby's reportedly getting out on Feb. 20, and will immediately report to halfway house in Los Angeles. She says the dates aren't set yet, but says she's been a model citizen in prison, and she's looking forward to her new life on the outside. Prison officials tell us ... as of now, Abby is still scheduled to be released on June 21.