Jillian Michaels Says Kate Middleton Doesn't Look Too Thin in New Video

Kate Middleton came across as somewhat thin to some who saw our new video of her from this past weekend -- but she's got one fitness guru in her corner ... one Jillian Michaels.

One of the first takeaways many had upon viewing the clip of Kate and Prince William at the Windsor Farm Shop -- about a mile from her home -- was that she looked sorta skinny ... or rather, skinnier than usual. KM is already a slender woman ... but here, perhaps more so.

To that, Jillian tells TMZ ... Kate looks just fine in her eyes -- both in terms of weight and overall health ... especially when you consider she went under the knife only 2 months ago.

In JM's eyes, Kate is right where she oughta be for a woman of her general size -- even noting to us that someone who's usually a bit thinner might lose some pounds while recovering after a procedure like the one she underwent ... and to Jillian, Kate looks normal.

On the subject of speculating on Kate -- be it her health, her weight or anything else -- Jillian tells us folks need to piss off and leave her alone ... 'cause it's not their business.

She reminds us someone's health journey is very personal -- and considering the type of microscope Kate's been under lately ... Jillian says everyone needs perspective.

Sage advice from Jillian -- but something tells us the public will continue picking apart every little detail. Remember, prior to the video TMZ published Monday ... we hadn't seen her on her own two feet since December, which is quite a long time.

Of course, she underwent abdominal surgery -- for a condition that hasn't been made public -- and Kensington Palace said she wouldn't be resuming her duties until at least after Easter.

What made everyone even more suspicious of her whereabouts and well-being had a lot to do with a photo she released on Mother's Day last week ... which was proven to be doctored.