When they showed close ups of her standing or running briefly, they had heelswhen she was jumping or fighting or doing a lot of rigorous activity (mostly when the stunt double was being used) they became flats all of the sudden.
The same thing happened with Malin Akerman's stunt-double when she played Silk Spectre II in Watchmen (2009)!
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The Tenth Doctor explains all.

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The costume malfunction actually happened to stuntwoman Jeannie Epper. She performed a stunt and her top came down, flashing everyone on the crew. After that, her top was modified to have flesh-tone straps, which were hidden by the camera distance, in stunt shots. She talks about it in the documentary, Double Dare, which features her and New Zealand stuntwoman Zoe Bell (of Xena Warrior Princess and Death Proof). The documentary also starts with a behind-the-scenes look at shooting a Wonder Woman stunt, where Epper jumps through a skylight. You can see the straps on her costume, as she speaks to either the director or stunt coordinator.
Fortunately, Ah keep mah feathers numbered for just such an emergency!

I did, among other things!!!!
Especially on those scenes where she was suspended upside down with potential for costume malfunction I watched her boots. Unfortunately any malfunction would only be available on a blooper reel and we the viewers would probably never hear or see it!
Someone somewhere did a great job of ensuring Lynda stayed cinched up tight in that costume so the twins stayed in place.

Who ever noticed her boots?

I would say she's stoickier. Lynda Carter has skinny arms and legs. The body double/stunt double was muscular and had broad shoulders that tapered to a thin waist. Whenever we watch an episode, I can't help but say, "Body double! That's Lynda Carter! body double! Double! Carter. Carter. Double!" (and so on)
I'm just expressing my opinion.
You may all go to hell, and I will go to Texas.

I wouldn't say "much stockier;" but, they had different body types. However, Jeannie Epper is a legend in the stunt industry, and she also doubled Lyndsey Wagner on the Bionic Woman.
"Fortunately, Ah keep mah feathers numbered for just such an emergency!"

What's funny is picking out her stunt double who is much stockier than Lynda Carter.

This also happened on "Isis". I recently watched an episode of that old show and she had high heels but when running the heel disappears lol

Nah, she was just so strong she pounded her heels flat, then slapped on a new heel when she stopped.
"Fortunately, Ah keep mah feathers numbered for just such an emergency!"

Yup, her boots would conveniently (and magically) change from high heels to flats depending on her activity.
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