I don't think so. Forums tended to suggest a size increase much better than +1 size per +1 mag, due to the typical increase in Target size being a x10 multiplier in mass. Aim for +1 mag is +3 size ranks.
i.e. a Terram increase and many other forms grant 10x times more material, or allow you to affect 10x more material. Also consider in that the rule which says that each 3 size ranks increased is a tenfold increase in mass. Combined that makes for each +1 mag equaling +3 size.
ArM5, p.18: "Every three points of size represents a ten-fold increase or decrease in mass, ... "
ArM5, p.113: "A base Group contains about as much mass as ten standard Individuals of the Form. This can be split up in any way desired, so it could be two Individuals, each of five times standard size, or ten Individuals of standard size, or ten thousand individuals, each one thousandth of standard size. Every five levels added multiplies the size affected by a factor of ten."
https://forum.atlas-games.com/t/the-break-room/102/1
I've created custom spells based from this interpretation and used then in sagas without concerns. Remember that the flexibility in the transformation needs to be paid for too, like Preternatural Growth and Shrinking pays extra for altering size up and down in the same effect.
A few folk have commented that they prefer less of an increase per mag, and created house rules for it. Its the old YSMV thing.
Can't help too much with the other stuff; except with unfounded opinion.