Michael Strahan's not holding up his end of the child support deal he struck with his ex-wife, and it's going to take him more than half a mil to square up ... at least according to her.
Jean Muggli claims in legal docs ... the former NFL superstar's on the hook for some big-time equestrian expenses for their twin teenage daughters -- Sophia and Isabella -- along with owing hundreds of thousands in back child support.
Jean says a huge chunk of what Strahan owes her is to cover their daughters' horseback riding bills .