there doesn't seem to be anything about population capacity in them
On the contrary
From MAXSHIPSLOTSTECH0:
researchModifier
modifierType "ShipMaxSlots"
baseValue 100.000000 <--- This means you start with 100 fleet points
perLevelValue 150.000000 <--- This means researching increases fleet points by 150
researchModifier
modifierType "IncomePercLost"
baseValue 0.000000
perLevelValue 0.093750 <--- This is the -% income for research
Each MAXSHIPSLOTS research subject increases "ShipMaxSlots" by some number of the previous research.