Actually, this is the more problematic part for a few reasons. The other way is more of a judgement call and harder to quantify, sure. But consider these:
The rules for Aegis of the Hearth combined with other canon books require size not to be part of R/D/T since those cannot be altered. This is because there are two canonical covenants with boundaries significantly, like 200-300%, bigger than allowed by Boundary, not just a little bigger like 20% that might be rounded away.
Meanwhile, all the rulebooks containing extra magnitudes for size agree that extra magnitudes for size do not change the Target. Look everywhere and the Target is still "Individual" or "Group" or similar, not "Individual +1" nor "Group +1" nor similar. This is in agreement with the issue surrounding Aegis of the Hearth.
However, many people consider extra magnitudes for size to be part of Target, which is not unreasonable because they modify Target specifically. If extra magnitudes for size are part of Target, then adding magnitudes for size would be the same effect with a R/D/T variation. So you are unlikely to find agreement here.
These drive me nuts. These are arbitrary punishments for the less-experienced or less-math-savvy player. Consider two players playing similar magi in the same saga. I have Magus A. A new player has Magus B.
Magus B invents these three spells:
Ball of Abysmal Flame
Ball of Dreadful Flame (CrIg 30) (Identical to above except its rules read "A ball of flame shoots from your hand to strike a single target, doing +25 damage.")
Balls of Terrible Flame (CrIg 35) (Identical to above except its rules read "Ten balls of flame shoot from your hand to strike up to ten targets, doing +20 damage each." and use Group.)
Magus A invents three spells:
Ball of Unfathomable Flame (CrIg Gen) (Identical BoAF except its rules read "A ball of flame shoots from your hand to strike a single target, doing +(level-10) damage.") This is invented at both levels 30 and 35.
Balls of Unfathomable Flame (CrIg Gen) (As the prior one, but using Group and so doing +(level-20) damage.) This is invented at level 35.
So, this other player and I have identical spells, varying in only two ways: their names and how we chose to write the damage, even though they do the same damage. We should expect the game effects to be the same, right? After all, I'm just an experienced player who likes formulas rather than a new player. But, alas, no. All of mine are similar to each other, so I get a bunch of bonuses for Similar Spells and General spells, while the new player doesn't. Why does the new player get punished? Because the new player doesn't know we place so much value on writing a formula versus writing the results the formula gives.
Think this is just a whimsical thought? Nope. We're even seeing it in canon with Wizard's Vigil v. Day of Communion.
This issue is exacerbated by the three different ways of writing out such patterns in the core book. We have cases (CrIg, MuCo, etc.) where specific values are listed as different guidelines. We have cases (MuIg, PeIg, etc.) where one specific value is listed and an adjustment is given for extra magnitudes. We have cases (MuAu, PeVi, etc.) where a single, general formula is given.
Some people solve this with only allowing same effect/General spells with "general" guidelines. First, this disagrees with General spells. Second, this creates a new arbitrariness. For example, the new player doesn't know that CrAq attack spells provide tons more Similar Spell and General spell bonuses than do CrIg attack spells. "Oh, sorry, you should have known when creating your CrIg Flambeau even though you never played before."