To reproduce:
1. Open attached project 'font-test2.zip'
2. Make sure GLES 3 is set as primary Graphics API
3. Build and run the project on Android

Result: Alpha8 textures are several times the size of same texture being ran on GLES 3 on Adreno GPU

Reproduced in: 2017.4.22f1, 2018.3.7f1, 2019.1.0b5, 2019.2.0a7

Note: results of attached example:
GLES3: 4.3 mb
GLES2 1.5 mb

Reproduced on devices:

VLNQA00093 Samsung Galaxy S6 edge 7.0 Exynos 7 Octa 7420 Mali-T760 OpenGL ES 3.2 05157df580154b28
VLNQA00178 Xiaomi Redmi Note 4 6.0 MediaTek Helio X20 MT6797M Mali-T880 OpenGL ES 3.1
VLNQA00096 Samsung Galaxy S8 8.0.0 Exynos 9 Octa 8895 Mali-G71 OpenGL ES 3.2 ce031713e23bccfa0d
VLNQA00057 Htc One M9+ 5.0.2 MediaTek Helio X10 MT6795T PowerVR Rogue G6200 OpenGL ES 3.1 build 1.4 FA58HYN00225

Not reproduced on devices:

VLNQA00024 Xiaomi Mi 5 7.0 Snapdragon 820 MSM8996 Adreno (TM) 530 OpenGL ES 3.2
VLNQA00001 Google Pixel 2 9 Snapdragon 835 MSM8998 Adreno (TM) 540 OpenGL ES 3.2 HT77E1A00039
VLNQA00015 Samsung Galaxy Note8 8.0.0 Snapdragon 835 MSM8998 Adreno (TM) 540 OpenGL ES 3.2 ce071717744900260c7e
VLNQA00257 Sony Xperia XZ Premium 8.0.0 Snapdragon 835 MSM8998 Adreno (TM) 540 OpenGL ES 3.2 CB512FYP25
VLNQA00144 Samsung Galaxy S9 9 Snapdragon 845 SDM845 Adreno (TM) 630 OpenGL ES 3.2