I don't know about wasting panel. Same hole, of course. Big Mom's body has already been under the magma chamber long before Kaido.
If Kaido's clothes burned the moment it touches magma, then it follows Big Mom should have been floating naked. If that is her body, it means she has been floating around like ± 30 minutes before Kaido, before Luffy turns G5. Her body, should has already been pulverised by magma (unless her body is magma-proof). Indicating the silhouette is just her chunk of body left after being consumed by lava (which is brutal). Why is the silhouette just her part of body, not whole? Because in the silhouette, Big Mom's body appears lower in height than Kaido's. While Kaido's height (710cm) should have been lower than Big Mom's (880cm) in actual. This is, if the silhouette is Big Mom.
You might say there is her feet and her right hand. But in the middle of her feet there should have been no clothes left. If it is indeed Big Mom's body, her clothes should have been gone, just like Kaido's coat.. But in the silhouette it indicates her Wano clothes are still intact (if she is not naked, it appears in the middle of her feet there is her Wano kimono), meanwhile Kaido's coat and clothes is gone. The logic doesn't follow from Big Mom's to Kaido's clothes.
This is the shape of Big Mom, for clarity I include it here. In the silhouette her head is gone, only a right hand and her feet pop out. Remember her height is 880-710=170 cm higher than Kaido.