Dude the whole protein doesn’t pack and interact - it’s the little protrusions and pockets that interact, very small parts of the whole.
Small molecule example was just to make things very simple - anyone can add to 28.
Oxytocin and vasopressin - closely related - close in size and weight, fairly similar in shape - different effects in the body.
Dude that simple molecule mention was still a stupid example, and you still are drifting with your last example. Dude.
Here’s a few questions, based on your last example. Let’s see if it drifts or is anchored, eh? What systems in the body are effected by, or process oxytocin and vasopressin similarly? Is there any correlation between allergies to vasospressin and adverse reactions to oxytocin, or vice-versa?
That is to say, may seem to me to be overly focused on what you hold to be the singular main action of these complex molecules — the active site (or sites) — yet the body have many processes that deal with each molecule, and adverse reactions can occur at any of those.
Yet something as coarse a characteristic as weight or size can alone be the defining or triggering aspect of some systemic process or pathology in the body. Say for example — the size of LDL, very much a part of how bad it is in clogging arteries.