Banner textures affected by a resource pack cannot be above or below the original texture resolution, unlike most other entities. This disallows creative flexibility with resource packs that may want to enhance, alter, or stylise the banner textures in any resolution smaller or greater than 64x64.

Attached are images of the banners in their regular form, with a custom texture at original resolution, and the same edited texture scaled 2x using nearest neighbour/pixel-perfect scaling, as well as the same sequence as seen in the loom pattern menus.

This is an issue with:
-all banner textures seen on banner entities
-all banner textures seen in loom UI
-all banner textures seen in other GUI forms

This affects versions:
-All Minecraft versions that include banners

RESOLUTION UPDATE:
To have properly scaling textures, you must resize assets/minecraft/textures/entity/banner_base.png as well as all files in assets/minecraft/textures/entity/banner/