When we create a normal portal in the overworld with no corners filled with obsidian, enter the nether and we will see that all corners are filled with obsidian. Same with big portals; if we create a big portal(suppose 8 by 8) in the overworld, the portal in the nether will be the default nether portal. If we build a default nether portal and enter the other dimension and break the corners off the portals, the corners would still be there in the previous dimension.