Assalamo alikom wa rahmato Allah wa barakato
What I know, is that we should open our legs up to the shoulders distance. (Sorry I don't have any evidence with me just now). I guess that some people spread their legs so wide is because they're trying not to leave any gaps in between each person, to follow Prophet Mohamed's (SAWS) recommendation.
Ibn Umar reported that Allaah (AWJ)'s Messenger said, Establish your lines [straight], for indeed the angels [pray] in lines. Align your shoulders, close the gaps, yield your hands to your brothers, and do not leave any gaps for Shaytaan. Whoever connects a line, Allaah maintains him, and whoever breaks a line, Allaah cuts him off. (Ahmad, Aboo Daawood, an-Nasaa`ee and others. See Saheeh ul-Jaami no. 1187)
Allah know best
Assalamo alikom wa rahmato Allah wa barakato