I'll just say that armor or not armor, size or not size, means nothing for Servants. For example, Mordred's armor is way bulkier than Baldwin's and she's a Saber, on the other hand, Diarmuid as Saber would still dress in light gear as seen on F/Z flashbacks where he fights with his swords. Lancers vary even more wildly (unless their are Irish, then obviously they gotta dress skintight suits of egregious appearance if you're not a woman), even Vlad has two impossibly distinct canon appearances and obviously lest not forget the walking gold statue that's Gilgamesh and still he's Archer.

Even much better warriors like Cu Chulainn and Diarmuid had difficulty in pointing their foes classes during combat. It's easy to narrow down to a few possibilities, but saying it's this or that outright is a bit off putting. Now, I'm not trying to sound like nagging or bullying, this is just my opinion on the matter. Whatever happens is good on my end, specially since my characters are not directly involved.