what exactly does "Reskinning your origin" look like, can you give an example or two?
An example stolen from a post on a different forum, long ago (or at least what I can remember of it):
"I really like the ideas behind the genasi race, but the story that makes most sense for my character is a member of dwarf nobility. Would it be okay for me to play a dwarf 'princess' of sorts, someone with special earth powers? I'm fine with only getting genasi-specific options, which we'll have to talk about to make sure things continue to make sense going forward. So, height, weight, overall appearance, all dwarf, but for everything that really matters in rules terms, an earth genasi."
For an example not using any official race for
aesthetics but using an official one's mechanics:
"Man, I've just rediscovered how much I loved GW2. Could I play a gnome artificer, but look and sound like an asura from that game? They're pretty similar culturally, and both are small. It'll just be a lot easier to find artwork that looks like what I'm imagining."
Or perhaps:
"I was hoping to play a Lizardfolk monk, but I really like the stuff you've done with the Imperial culture and the dragonborn in it. Could my character be seen as a sort of 'feral' or 'jungle' variant of dragonborn, one that doesn't get a breath weapon but develops in other ways?"
Any situation where a particular aesthetic is desired, but the mechanics are consistently taken from a thing that doesn't have those aesthetics. As noted, I allow for the notion that "creature size" is something that exists on both an important-to-the-mechanics level
and an important-to-the-aesthetics level, so I could totally see a DM arguing that reskinning cannot change a creature's size. So you could play a gnome with halfling stats or a goblin with kobold stats or whatever, but you couldn't play a goblin with loxodon stats or an elf with gnome stats.
Edit: My examples were chosen to illustrate that the reason for this switch could be mostly roleplay-oriented (the asura), mostly mechanics-oriented (lizardfolk), or somewhere in-between (the dwarf princess). And I 100% grant and agree with the "don't do this to grub for benefits" kind of thing. Any player using this as a pretense for squeezing the maximum mechanical benefit out of stuff is acting in bad faith.