Hi @dan.studio55-jp,
Mobile devices have much less RAM than desktops: usually 2GB, but sometimes only 1GB, and this limits the size of scenes which can be opened on the most common mobile devices. (The iPad Pro you’ve mentioned has 4GB RAM, and this is why it handles the scenes the iPhone/iPad do not).
In most of the cases it’s possible to make a scene work on common mobile devices by decreasing the number of triangles and lightmaps in the scene. You can find some general advices in this post: How to two or more walkthrough if project is big? - #2 by jan , but if you shared a link to one of your large scenes, we would be able to give you more concrete feedback.