Custom photo sticker sizes vary on iPhone, iPad, and apps

I don't know why, but some custom stickers made from my photos are smaller than others, and even among the smaller ones, the sizes vary. This has been happening since the early versions that allowed sticker creation on both my iPad and iPhone. It also seems like the sizes appear differently depending on the app I'm using. Please help me with this! Below is a screenshot showing the stickers on two messaging apps, Messenger and Zalo. As you can see, the sizes also vary between the stickers themselves.