There is Forvo German Dictionary that weighs 4.3 GiB and works perfectly on DictTango:
https://forum.freemdict.com/t/topic/7355
However, an English Pronunciation Dictionary that weighs 3.1 GiB does not work.
There might be another reason besides the 2GiB “limit” because Forvo .mdd works and is larger (4.3 GiB) !
Maybe the problem is related due to the fact that MDict format is closed-source and the different versions of the MDX builder might have different standards.
Maybe it is a “compatibility issue” of DictTango and a specific .mdx/.mdd file ?