Brunhilda is a figure from Norse myths and legends who has somehow come into popular culture as a really fat woman. In fact, thanks to Wagner, she IS the singing fat lady of the saying. And while I certainly don’t mind plus-sized princesses, I’m not really sure how this image came to be. Traditionally, Brunhilda was a Valkyrie who lost her position because she decided the outcome of a battle in the opposite way from what Odin wanted. As punishment for her radical free-thinking, the ruler of the Aesir forced her to live as a mortal, and trapped her inside a ring of fire (and it burns, burns, burns) in a castle in the Alps. The great hero Sigurd or Siegfried found her and rescued her, promising to marry her.
A potion given to him by the sorceress Grimhild made him forget this promise, however, and instead he married Grimhild’s daughter Gudrun, also known as Kremhild. Grimhild’s son Gunnar decided he would marry Brunhilda, but was blocked by another wall of fire, so Siegfried had to disguise himself and rescue her again. When Brunhilda found this out, she was angry and conspired to have Siegfried killed, and then jumped onto his funeral pyre and died herself. The Niebelungenlied, the main source material for Wagner’s Ring Cycle, makes Brunhilda into the Princess of Iceland and has Siegfried battle Brunhilda while invisible rather than riding through magic flames, but the basic story is the same.
There was also a historical Brunhilda, on whom the mythical character might have been based. This woman was a Visigothic princess in the sixth century who married King Sigebert of Austrasia, a Frankish kingdom. After Sigebert was assassinated, she took over as regent, instituting many reforms in the kingdom. Later in her life, however, she became ruthless and bloodthirsty, plotting the deaths of many of her political rivals. Reports say that she was eventually killed by being torn apart by wild horses, after which her body was immolated.
As Sigebert had a brother named Guntram, those are at least three names that are similar to those in the legends.
Brunhilda definitely sounds like an infamous enough figure to have passed into legend, and her bloodthirsty ways in her later life probably contributed to her being seen as a war maiden. I can only guess that the stereotypically masculine traits that came to be associated with her were what led to her rather ample figure in more recent media, but I can’t be sure.