I have my flight ratios set to 100% medium aircraft. Despite that, some airlines are still offering flights involving small aircraft. Although the screenshot is only showing the list for 1 airline, several airlines are doing this.
Hmm, no it doesn’t sound right. I mean it will not remove any already generated flights but if you set the small aircraft value to 0, it should not generate any new ones at least. I will have a look at this to try and find the issue. Thanks for reporting. 
Currently the longest flight by budget airline is Eurowings (owned by Lufthansa) from Cologne to Bangkok (Around 11:35 hrs).
Norwegian was holding the record for its flight from Oslo to Bangkok (11 hrs), Copenhagen to Los Angeles (11:20 mins) for sometime.
There is a new airline called Level (Owned by IAG group) which is operating flights between Barcelone and Los angeles , Barcelona and Buenos Aires.
By Next month end, the orders will change and might look like this :
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Norwegian, London-Singapore, 10,841km, Outbound time: 12:45, Inbound 13:40, Aircraft: 787–9
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Level, Barcelona-Buenos Aires, 10,459km, 13:45, 12:45, A330-200
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French Blue, Paris-La Reunion, 9,386km,10:55, 11:40, A330-300
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Eurowings, Cologne-Bangkok, 9,063km, 11:25, 12:15, A330-200
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Scoot, Singapore-Athens, 9,042km, 11:30, 11:25, 787-8
Some background:
There were times when Budget/Low Cost airlines used only single aisle planes (like Boeing 737 and Airbus A320 family) and used them for shorter flights (1 to 3 hrs). Now these planes are used for 4 to 6 hrs of journey time like Southwest, Spirit, EasyJet, RyanAir, Air Asia, Indigo, JetStar etc…
But the trend has changed and people wanted to fly long distance on Budget airlines.
So now there are many airlines using Twin-Aisle aircraft Like 787 dreamliner, A330. With the dreamliner, Airlines are able to provide cheap base fares and still make profit on long haul flights. Example Scoot, Norwegian Air Shuttle and Jet star.