They seem like a good size since they are the rights size for 1-week old chicks.
Here are some problems that could be causing the sudden deaths:
1. According to you, what is hot weather, 20 degrees Celsius or 30 degrees Celsius? Chicks of one week old should be in an environment with the temperature of at least 30 degrees Celsius. Therefore, maybe your chicks are too cold, you can add some heat lamps or warm water bottles into their run.
2. They might be dying from the wind, chicks can often suffer when there is constant wind. Air circulations cool down the chicks which they can die from.
3. Pasty buts. Your chicks seem to be eating and seem very happy, but they may have poop stuck on there but which will stop other poop from getting out of their system. If chicks have pasty but for a period of time they can die due to waste getting stuck in there system. Read more about it in this article: http://www.raising-happy-chickens.com/pasty-butt.html
4. The chicks could be overheated meaning that it is too hot where they live. You can prevent this by providing shade or by putting frozen water bottles in their run.
5. Virus, disease, etc. Your chicks could be suffering from a harmful bacteria that is your area. The bacteria is not visible to your eye but will kill the chickens. To stop the bacteria you can try and put some sore of medicine in their food or water. They could also be suffering from mites, lice, etc.
6. Finally, their parents may come from a bad breeder which could cause very unhealthy chicks and therefore sudden deaths.
What are you feeding them? See how they're all bunched up together by the crate? They need stable temperatures day, and night with no wind, breezes, cloudiness, etc. causing temperature changes. Week 1 = no less than a constant 90 degrees. Week 2 = no less than a constant 85 degrees. Lower the temperature 5 degrees each week, until they are feathered out.
From his user name and the building materials seen in the video. I'm going to make a wild guess that he's in or around Egypt. If temps are anything like they currently are in Cairo they should be plenty warm during the day and only slightly colder at night. Perhaps he needs some heat at night, although with that many chicks he's bound to have some losses regardless