I'm no expert on these, but seeing that no one else has chimed in, here is what I know about the Lionels. Like many things Lionel, the explanation is in Lionel history, not railroad history.
During the prewar years, Lionel made their traditional-sized, streamlined K4s in both O and O-27 versions. It is not obvious in photographs, but it is easy to see in person that the O version is larger overall, and significantly wider than the O-27 one. Both versions used a box-shaped sheet metal tender, but again, the tender for the O locomotive is larger and wider than the tender for the O-27 loco. The streamlined K4 did not return after the war, though the O-27 tender was found behind the 221 and some of the early Scout engines.
In the 1990s, when Lionel re-issued the traditional sized streamlined K4, they used the casting from the larger O gauge engine. But rather than pair it with the prewar box-shaped tender, they chose the streamlined tender, which had originally been made for the 671 turbine. This tender is longer and better looking. The choice might also have been economic: I suspect that Lionel would have had to outsource a sheet metal tender (as they were doing with their Classics line), when they were already making the 671 tender for other locomotives. So the engine-tender pairing was a compromise; speaking for myself, I feel like it works.
What MTH's excuse is, I have no idea!