Where does this idea that Engineering is the largest tech tree come from?
Society is the largest of the disciplines, having far more techs than any of the two others, and to make matters worse it its heavily frontloaded being especially numerous in the early tech tiers.
Of core techs (the base 00_<discipline>_tech.txt files), society has 138 distinct techs that aren't disabled, and engineering has a mere 84. This is why 00_soc_tech.txt is 100.647 bytes long compared to 00_eng_tech.txt being 54.400 bytes long. (And 00_phys_tech.txt is a mere 39.018 bytes long.) There are a few - a very few - in the core that are mutually exclusive (e.g. biological vs machine version of same tech), but not enough to change the picture.
Likewise all the DLC and other tech files taken together doesn't do much to change this general picture, because most DLC has either a few techs that aren't mutually exclusive or no techs at all, and the only two that have many that aren't mutually exclusive are First Contact (12 physics, 6 society, 4 engineering) and Megacorp (Megastructures adds 6 physics, 4 society, and 2 engineering)
The Society tech tree is massive compared to both engineering and physics, which is why players who pass up on important early society gateway techs like Planetary Unification risk greatly hindering their long-term development because the sheer number of techs once they break into tier 2 means that even common techs aren't that common until your science investment is large enough that you are churning society techs fast.