Thanks for the hint, I'll try it. Just voted in your request. Most likely, for more correct results, for now we'll have to use a Blender or 3D max (maya).

Hello,

Sadly we can't animate the body scale in the timeline. It's been something I've requested since I work on iClone 7 in september 2019 : Change the size of a character in the timeline - Feedback Tracker (reallusion.com)

I encourage you to vote the new issue I made for iClone 8 in june 2022 even if I'm loosing hope about that feature : Change the size of a character in the timeline - Feedback Tracker (reallusion.com)

For the moment you can use some tricks :

Thanks a lot for the answer! Yes, I also really hope that such an opportunity will eventually appear.

Hopefully one day iClone would be able to do that, but not at the moment.
Some limited, cryptic workaround: https://forum.reallusion.com/FindPost500521.aspx

Hello everyone.

I haven't been able to figure out for some time whether there is an opportunity in Iclone to animate the scale (or growth) of the character's body? Let's just say that initially the character is small (normal, standard), and after that he should increase in size and get musculature. How to make a character muscular is understandable, but how to scale the character's body itself is not entirely clear. Because the "body scale" parameter in Iclone is not animatable at all. 

Perhaps someone has encountered and solved the problem of animation of growth \ scale...