Does it mean that the animation in binary have no certain size(border) at all like regural video or gif? It only contains the infromation about bone movement?
Rather than having no borders, it would be better to think of the bounds as being variable to match the pose while the animation is playing. The method of determining the bounds depends on which runtime you use and how your skeleton is implemented. For example, in the case of Spine Web Player, by default the bounding box is automatically calculated for each animation, but it is also possible to specify the viewport size and trim any animation outside the size:
Spine Web Player: Viewports
Also, you can scale skeletons at runtime to display larger or smaller sizes. Please see the Spine Runtime Guide for general information of Spine runtimes:
Spine Runtimes Guide