This thread made me think too much…
I think this is likely a result of the units designed for said classes, typically gba designed the generic class animation to vaguely match the characters that have that class while still being generic enough to use for all the enemies. [except foe women because no female enemies exist in gba FE]
however I think the main reason is just to make the sense of growth for the unpromoted units feel more impactful.
most mages are children [or at least young] while sages tend to be older
this lines up with fe7 where Erk is in his late teens, but pent is older, Fe 8 also lines up with Ewan/Lute and Saleh, I’ve always thought of the Mage to sage promotion as a metaphorical growing up of the mage, the larger animation informs that.
A similar philosophy is seen with the fighter and worrier, fighters a typically just guys, strong guys but just guys so they are rather plain and unimposing, this is also [although more so of brigands] is to make them look weaker and closer to your units early on, that is to say less intimidating, when fought in the early game, Warriors and zerkers are usually still guys but imposing muscular guys that are build like a fortress see fe6 bartre, or Garret, they are big guys that are more imposing than Wade or Lot are, as such the unpromoted have to train up to that strength.
Generals are meant to be big imposing walls of Armour so their big and imposing, they also serve as important bosses in FE6, while armour knights mostly lower rank troops that are lesser than the general, honestly this one can just be explained by the unit putting on more armour after promoting to a higher position.
I think the myrms are the only real contender here for being notably odd, however I’d say it’s just to make swordmasters look more powerful/imposing than the myrm, while still looking sleek and being easier to animate with the plain design.
The rouge and assassin are designed after specific characters Rennac and Jaffar respectively as such the fact others can use these classes is mostly an after thought as such there is little need to design around them, especially as FE8 made sprites more detailed redesigning the thief was probably seen as not worthwhile.
The classes that are listed as same height do the same thing but in different ways.
lords, are the main characters as height consistency would be more prioritized due to specialized class and animation being reflective of them much more closely.
Snipers have a massive bow meant to out shine the archer’s more reasonably sized one, in a way mimicking the size growth of the other classes, this is even seen in nomads troopers and rangers.
Heroes get the drip and a shield to display their superior equipment, this aspect is also displayed in their portraits being more equipped than mercs, look at Harken compared to Raven.
This trend is also seen in mounted units Paladins have shields, a helmet, their horses have more armour and are draped in horse cloths? [I know noting about the correct terms], the armour aspect is even more prominent in great knights.
Really in gba promotion does usually mean an increase in size for a unit, this is also reflected in the con stat increasing, it’s a reflection of growth which is really what FE is all about.
On this note for gba I don’t think so unfortunately, however It may be possible archer to ranger or one of the horse lords do, and Amelia to Armour knight might if you account for the armour.