The OBB is likely so large because you have packaged for Android which cooks and packages all texture formats. Choosing just one texture format (ETC1 is supposed by all Android devices) will reduce the size and cook/package time.
1.75 gb is a large OBB; it is possible this may be the issue. It may have failed to installed or found at startup.
Really need to see the logcat from the device at this point if possible.
Thanks!