A.T. Robertson Commentary Mark 16:4

"and looking up, they see that the stone is rolled back: for it was exceeding great." — Mark 16:4 (ASV)

Looking up they see (αναβλεψασα θεωρουσιν). With downcast eyes and heavy hearts (Bruce) they had been walking up the hill. Mark has his frequent vivid dramatic present "behold." Their problem is solved for the stone lies rolled back before their very eyes. Luke 24:2 has the usual aorist "found."

For (γαρ). Mark explains by the size of the stone this sudden and surprising sight right before their eyes.