Jessica Simpson -- I'M HOT AGAIN ... Goodbye Mom Jeans, Hello Leather Pants!

Bad news for chubby chasers ... fat Jessica Simpson has been replaced ... with a hotter, skinnier version.

The 33-year-old mom strolled though JFK this week like it was 2005 all over again ... sporting a pair of tight leather pants that would even make Nick Lachey's head spin.

Despite her appearance, Jessica recently said she's currently 15 POUNDS HEAVIER than she was back in 2009 ... when she rocked the infamous mom jeans on stage at a concert.

Sure as hell doesn't look like it.