It's likely that many of the writers of older texts just "assumed" that adult pythons (Morelia) eat frogs and reptiles, just as many of the writers and taxonomists in the 40s & 50s assumed that smaller snakes ate insects. (The texts which were available to me in those days were very misleading because nobody did anything but catch things and put them into jars of preservative - no stomach content analysis, no reproductive studies or anything like that).
Many snakes, and especially GTPs, undergo a niche change as they grow - they occupy a different part of the habitat as they get bigger. The heat pits in Morelia (not present in Aspidites which have a large proportion of their food as reptiles) say to me that these things were actually meant to live most of their lives eating warm-blooded prey (mammals, birds), so once they are large enough to overpower mice, rats birds or whatever, that's their preferred food because it's what they are equipped to deal with. Very small pythons cannot overpower these food items, so they aim lower, at frogs, skinks etc. Baby pythons would rarely, if ever, come across pinky mice in the bush, and I'd doubt if they would eat them if they did, in the first few months of life. Our use of these food items to raise hatchies often makes the starting of a feed response more difficult, because it is very foreign to a small python... but it's the only option we have sometimes. Put a suitably sized skink or gecko in an enclosure with a small python and the fun begins immediately - hence the scenting with lizard or frog scent to get difficult starters feeding.
There may be some trigger which causes the change at a certain point in their growth.
Jamie