Folke den tjocke, jarl av Östergötland - Folke the Fat (Swedish: Folke den tjocke), according to Gesta Danorum?

My personal guess is that diger / digre in his name signified "the great/mighty ". Perhaps he was both mighty and corpulent.

What was the wording in earlier Swedish sources?

It would be interesting to see when the wording "the Fat"/"den fete" started to be used.

It is indeed good that you changed the name in the geni-heading

Personally I think theres no need to interpret the sources. If he was called Digre in the source, that the term which we should use. Not interpret it to what we Believe it means. After all, genealogy is not about guessing games, its about citing prime sources. Apart from this, I think someone has guessed about his COA, I dont think one single source support the claimed Coats of arms.

I agree with you there. It's good practice to stick to the original source.

My reflection was merely about that other people have interpreted word "digre" as just "the fat", "den tjocke", i.e. the voluminous or even the obese.

Looking at the power position he must have built it might as well refer to his wealth and or power. So it would be better to stick to the word "digre".

Carolus Magnus would become Charlemagne, Karl den Store, not Karl den tjocke....

When anglosaxons write about the early nordic persons the names are often translated into something that changes or adds new meanings.

For example:
Tore Teiande -> the Silent, (the Taciturn might be a better translation)
Olav Bitling -> the Dwarf, the Morsel, the Crumb - while it probably had a totally other significance, we will never know. So Olav Bitling would be preferred.