2014 rear sub size from the factory
 

Will simply swapping out the factory sub for a higher quality one solve the weak bass issue on my 2014? What size is it?

technically no. the issue is the "360"w amp. it is way underpowered.

u could possibly notice a slight improvement but imo, youre just wasting money.

if it is the same as my civics ive had(which it probably is) 8"

But swapped out and connected to its own power amp would probably work, correct?

yep. if kept in the stock location, may have to fix rattling issues.

i would suggest sound deadening on the metal deck and some ccf or insulation sandwiched between the metal deck and the plastic deck when it gets re installed