Rooms of Japanese hotels are relatively small.
On the other hand, amenities and equipments are much more than hotels in Europe. Toothbrush, toothpaste, razor and comb are located in the bathroom and usually you can find a device which has function of humidifier and air purifier in the bedroom. Some better hotels are equipped with trouser press.
Then, Japanese hotels have different rate systems than hotels in Europe. Usually hotels in Europe charge per room. If you book a double room, the rates for staying alone or staying with your partner is small difference. However, hotels in Japan usually charge per person. If you stay alone, may be you pay JPY 5000 per night but if you overnight with your partner, your bill will be JPY 10000 per night.
Which means twin room(2 single beds) is usually double price of single room(1 single bed). However, double room(1 king size bed) is not the same rate as twin room. It is usually cheaper. Then, hotels also offers semi-double room(1 queen size bed) which is cheaper than the double room.
Then, the biggest difference between Japan and Europe is that hotels in Japan do not have many double rooms. Many hotels have plenty of single rooms for their business clients, and twin rooms for tourist clients. Japanese do not prefer to stay in double room, especially they are not young couples or on honeymoon.
Anyway, please be careful when you book the hotel in Japan. There are significant difference between double room and twin room, the bed type and the rate.