How to deal with person pinching legs together

When playing guard against some larger opponents, they sometimes pinch their legs together whilst i have a foot inside which not only immobilises it but makes it impossible to pummel my other foot in for SLX. I dont mean that they're high step passing as theyre facing me head on.

I have a feeling that by narrowing their base this much there's some obvious counter that i havent realised - do i just push them back to force them to post with their foot and open up?