The real problem is not volume alone but mixed access frequency
By late August, many travellers start carrying light warmth, light rain protection and backup footwear together. The mistake is often not the item choice but mixing all of them into one layer.
On a Xinjiang road trip, the key question is whether changing weather can be handled without turning the vehicle into a luggage search every time.
The cabin should hold only high-frequency or true response items
A steadier setup is to reserve cabin access space for the items that may genuinely be needed during the day, such as light rain gear, one easy warm layer and a pair of shoes likely to be swapped quickly.
Once the cabin starts holding every possible maybe-useful item, it stops serving the things that need to be reached fast.
Heavy coats and low-frequency spare shoes often belong in the main layer
If a coat is mainly for early morning or evening, or a shoe option is only for selected days, those items often work better in the main-luggage or hotel layer than beside the seat every day.
That keeps daily cabin order cleaner without ignoring weather changes.
Separate the weather-response layer from the hotel-reset layer
The strongest late-August packing system is usually organised by use scenario, not only by traveller.
The weather-response layer supports the road day, while the hotel-reset layer handles slower evening reorganisation. Once those layers are separate, both daytime access and nightly packing become easier.