The bug

For all textures in the game, you can change the resolution using a resource pack simply by increasing the dimensions of the file. In bedrock, this does not work for the following texture files:

• font/glyph_E0.png
• font/glyph_E1.png

Doubling the size of these image just makes the characters physically larger on-screen. It's impossible to increase (or decrease) their resolution.
These two files comprise unicode characters U+E000 through U+E1FF, the beginning of the private use block.

It's incredibly useful for mapmaking purposes to add custom icons there, since they are designed for private use. However, this limitation means you instead have to infringe on more legitimate characters in order to increase your resolution.

Also note that it's impossible to test sheets like E2 due to MCPE-129711.

Important note

The current behavior is exploited by dozens of custom maps to draw large, low-resolution images to the screen. Fixing it without versioning the fix would render these irreparably broken.
The best solution in this case would be if bedrock had data-driven fonts like java minecraft does. Then you can explicitly choose both the scale and resolution of individual characters. But we all know that won't happen.

How to reproduce

1. Download the attached world
2. Look at the two signs
→ Despite both characters using an identical texture, the one from sheet E1 is gigantic