I haven't used pony gallery, but I'm asuming it has a .css file or .php file associated with it , in order to determine it's max height and width. It might even have a backend to specify it's max height and width. If it's a module then you should be able to find the files you need to modify in :
/modules
/administrator/modules
mod_pony.... possibly will be the filenames
if it's a plug-in - check in the backend portion of joomla components / community builder / plug-ins and see if there are any height / width settings.