Honorary mention to the airport whose English name is Osaka International airport but which is known to the locals merely as Itami airport; reflected by the airport code ITM. It is quite far from whatever you want to consider the city centre of Osaka (the Umeda area being closest yet still far away). It does, however, sit in the middle of the urban sprawl and there are no visible municipal boundaries between it and Osaka.
Furthermore, it is situated on the land of Toyonaka (396,000 inhabitants), Ikeda (103,000) and, yes, the ciy of Itami (197,000) – all three of which are larger than the 50k you requested in the question. The distances from the terminal building to the centres of these three (taken as the city hall or 市役所 of the respective cities) according to OpenStreetMaps (pedestrian) are 4.3 km to Ikeda, 3.4 km to Toyonaka and 5.9 km to Itami. (Itami City is across the runway from the terminal building. Wikipedia claims there be a long tunnel passing beneath terminal and apron to Itami city which would shorten the distance considerably as the current route detours around the terminal but if that tunnel still exists it has not been mapped out.)
These numbers give it a weird distinction of having two cities closer to it than the two different cities it is named after – and being well within walking distance of the centres of three different 100k+ cities including one of its namesakes.