[GeoffR]

Thanks for the extensive testing jackie_w! (I can't figure out why the original Aura uses the larger font scale though.)

I'll add this patch, with a new name, to the current firmware patches. As noted it should work for versions 4.6.10075 and later:
ePub uniform font scale (paste into libnickel.so.1.0.0.yaml)

Spoiler:

ePub uniform font scale:
  - Enabled: yes
  - Description: |
      This patch makes the ePub reader use the same font scale on all devices,
      the same as the KePub reader does. Without this patch the ePub reader uses
      a 50% larger font size on AuraHD/H2O/GloHD/Clara (and Aura?) devices.
  - FindBaseAddressHex: 00 00 C0 42 00 00 10 43
  - ReplaceBytes: {Offset: 6, FindH: 10 43, ReplaceH: C0 42}

The same method of increasing the font scale by 50% for some devices seems to be used in all firmware versions at least as far back as 2.8.1, but the way the data is stored is slightly different for different versions, and it might well change again in future versions.

This is a version of the patch for firmware versions 2.8.1 - 3.19.5761:
`ePub uniform font scale: (paste into libnickel.so.1.0.0.patch)

Spoiler:

<Patch>
patch_name = `ePub uniform font scale:`
patch_enable = `yes`
# Multi-version patch: 2.8.1 - 3.19.5761
#
## This patch makes the ePub reader use the same font scale on all devices,
## the same as the KePub reader does. Without this patch the ePub reader uses
## a 50% larger font size on AuraHD/H2O/GloHD/Clara (and Aura?) devices.
#
find_base_address = `\x00\x00\x10\x43\x00\x00\xC0\x42`
replace_bytes = 0002, 10 43, C0 42
</Patch>

If anyone wants this patch for a firmware version from 4.0 to 4.5 just let me know.