Backpack for small dogs

I found a brand that sells small backpacks for dogs in my country. They are basically the same as the large ones, with one pocket at each side.

Even though I loved the idea (always used backpacks with my larger dogs) i’m afraid it might be too much for their little bodies. Does anyone know if there is a size limit for wearing a backpack? At which age could i introduce a small sized dog to wearing a backpack (initially without any weight). All advice is wellcome.