Quite correct. But it’s the take-off performance which is the crucial matter as planes taking off from FLR have to cope with the nearby Apennines.
FLR-FRA is around 450 miles whereas FLR-LCY is around 750 miles. So although LH’s E190 can operate to FRA could that plane type reach LCY bearing in mind take-offs from FLR can be tricky on hot summer days when flights are full up with pax and luggage ?
Runway limitations at FLR dictate the plane type and length of flights that can be operated.