Why does parenting Heel IK to Thigh Bone cause jittering in right leg but not left leg? I have a character with legs that are not parented to a torso bone (they end at the top of the thigh - each thigh is a highest level parent). Other than that, I rigged the legs normally.

I wanted my character to be able to retract his legs upwards into his body. To do this, I scale each thigh bone down in pose mode.

This works for each leg, but each foot remains the same size, because each foot bone is parented to its heel IK bone.

To solve this, I tried setting each thigh bone as a parent of its respective heel IK bone. This worked fine in the left leg. However, in the right leg, this causes the bones to move wildly all about in pose mode.

To see what was wrong, I tried separating the right leg with all its bones into a different armature. When I did this, the right leg behaved normally.

My paint weights are fine, and influence the mesh properly; this problem is only with the armature. Both legs appear to have identical bone hierarchies, and identical constraints.

I've been able to solve my problem by setting a bone constraint to match each heel IK's scale to its thigh bone's scale. However, I'd still like to learn why parenting the bones the way I did caused the strange behavior, and I'd really appreciate help. Thanks!

I can post more information or screenshots if that would help.

Edit: https://drive.google.com/open?id=1-bWzTfC_T1BdgUn1wnHGU0S8ra1XRreM
To have an IK controller which is child of a bone which is part of the IK chain must be considered a rigging error, because it creates a circular dedendency, leading to unpredictable behaviours (and jitterings).

Instead you can create a new bone (which I called "scaler") that will be parent of the thigh and the HeelIK. In my picture scaling the scaler bone caused the whole left leg to scale proportionally, maintaining full functionality of the IK leg chain, without any need of constraints.