A lot of that is a size difference. I think you'll find that pygmy ankylosaurs hold up better against poorly-equipped, weaker enemies (goblins, lizardmen) than sabre-toothed tigers do. Pygmy ankylosaurs have scales roughly equal to bronze in strength, while sabre-toothed tigers just have their hides. However, when badly outsized, that advantage doesn't mean much. One on one, a sabre-toothed tiger will win about 75% of the fights with a pygmy ankylosaur, but against a copper-clad goblin siege, the ankylosaurs will last longer. Megalania are much more powerful and better armored than both sabre-toothed tigers and ankylosaurs, but come at the price of having to capture and breed them on your own.
A little size and ability reference chart.
Dwarf = 60k
Dog = 30k, weak defense, mediocre offense, embarkable.
Sabre-tooth tiger = 425k, weak defense, very powerful offense, breeds and matures quickly, embarkable.
Pygmy ankylosaur = 90k, strong defense (~bronze), mediocre offense, lays eggs, embarkable.
Megalania = 600k, very strong defense (between iron and steel), very powerful offense, lays eggs.
Empyrean carbuncle = 10k, very strong defense (~steel), weak direct offense, strong indirect area-effect offense (bladed air), lays special eggs, flies, cannot be reliably bred.
Damascus sentinel = 120k, extremely strong defense (damascus steel), strong offense, avoids traps, syndrome immune, drops t1 tech tree item on kill.
Devourer of the depths = 1000k, strong and valuable webs, avoids traps, paralysis venom, weak defense.
Wyvern = 25000k, strong defense (~iron), strong direct offense, very strong indirect area-effect offense (dragon fire), avoids traps, fire immune, lays eggs, flies.
Land mover = 5000k, strong defense (scales, ~iron) to very strong defense (shell, ~steel), mediocre offense, debilitating blood, avoids traps, lays eggs.
Damascus arbiter = 10000k, extremely strong defense (damascus steel), extremely strong offense, avoids traps, syndrome immune, drops easter egg item on kill.
Greater demons = 1700000k - 2000000k. Not a typo - imagine a dwarf fighting something the size of the statue of liberty. You'll have to figure out their abilities on your own.