Most cars have smaller batteries these days to save weight and because engines start a lot quicker than they used to. Having the hazards on for an hour will draw 4 x 21watts / 12 = about 8 amps, but because they flash it will be less than half that so 4 amps for an hour or 4 amp hours from a 35 amp hour battery, this suggests that your battery was either in a very low state of charge from very low mileage short journeys or more probably the battery is on the way out, is it more than 5 years old. I have never ever had a problem with battery on a Jazz going flat. When a car starts first turn of the key ( which our jazzes always did and still do ) it will take about 2% of a full battery charge.
Phone and dashcam would take very little power from battery.
The jazz battery is plenty big enough for its purpose providing it is in decent condition. Any battery over 5 years old is living on borrowed time, we are lucky in UK that our batteries last a long time, in hot countries like Africa, Australia and southern USA they can fail in less than 2 years.
I suggest you get a new Yuasa battery before the winter, otherwise you will be jump starting it a lot of times. It may be that boot light is staying on, anything bigger like heated screen or aircon relay stuck would flatten battery completely overnight.