Since Hazrat Mahdi (as) will have a wide stomach, his thighs will naturally be broad, as well.
His [Hazrat Mahdi’s (as)] brow is open... his stomach large, and the space between his thighs is open. ... (Mari’ bin Yusuf Karami Hanbali, Fawaid Fawaid al-Fikr fil Mahdi al-Montazar, p. 13)
THERE IS A SPACE BETWEEN HIS THIGHS... (Mar'i ibn Yusuf al-Maqdisi "Fera Idu Fevaidi'l Fiqr Fi'l Imam al-Mahdi al-Muntadhar)