Enter Air 737-800 flights leaving out of Skiathos JSI normally operate with split pax loads (Sunvil Holidays)
They fly LGW-JSI then on to Lemnos, Chios or Volos, then direct back to LGW thus a/c is light out of JSI and they fuel up at the next island.
Other 738 operators also do split loads at JSI -
Neos, Blue Panorama, TUI Fly, Smartwings, SAS, Transavia, all flying in to JSI then on to Mykonos, Karpathos, Chios, Lemnos or Santorini
If they do ever operate non-stop back to UK then usually flight will be not full, as both 73NG and A320/321 are payload limited out of JSI.
They will normally though Tech stop on the way home at places like SKG CFU PVK KVA VOL or occasionally ATH.
757 has no such issues operating in or out of JSI
Smartwings are using a 737-700 on Charter Flights to tiny Paros Island (1400m) from Prague, but fuel up at JTR for the return.
Payload limited to 120 pax (148 pax is full capacity)
737-800 and 737- MAX had been flying in and out of Jersey for a local Tour Operator to Spain and TFS in the recent past.
https://youtu.be/s2CYwP3hF0Y?t=17
UK - JER flights will likely not have loads of bags, nor need a load of fuel.
Jet2 maybe using their 737-300 which does not have too many issues in or out of JER