I think the explanation can he shortened to two words: visual hyperbole. It's like when Demonio is going into baby mode: they don't transform into babies, they just act like ones. But what it's funnier, a full-body late teen acting like a toddler or a SD baby caricature acting like one? Here Tierra was visually "transformed" to drive the point of how he was treating her.
I think in the cases involving Tierra mama, it's an actual 'temporary'
physical transformation into baby mode, not just figurative.
After all, this is a world of magic, so it's all good. Tierra mama probably has some special baby transformation spell which the MC reversed on her. You can see Pena is also questioning how it works.
Have to say the Tierra mama toddler mode is cute though haha!