What planets are they shown to have conquered besides Star Killer base in TFA?
We see a total of what 1 star destroyer which is Kylo Ren's / Hux's and Star Killer Base from the First Order in TFA. Hardly some extremely powerful group in the absence of Star Killer Base.
TLJ is where they suddenly have a giant fleet and own everything, not TFA.
None of that in either movie to be honest really made sense to me. We get no context in the TFA as to how things are post-ROTJ, it would be like having a LOTR sequel 20 years afterwards and Aragorn and company are back trying to battle a new evil rising for Mordor that has a massive army already but there isn't really any discussion about what specifically happened from ROTK to now.