To demonstrate the difference in write speeds in raw numbers, YouTuber Unbox Therapy uses a benchmarking app to run a real world test where he transfers  4.2GB copy of “Star Wars” from a Mac to the phone. The 256GB iPhone 7 finishes copying in 2 minutes, 34 seconds, while the 32GB iPhone 7 does the same job in 3 minutes, 40 seconds. That means it takes the 32GB iPhone 7 about 1.43x more time to copy a 4.2GB file as the 256GB iPhone 7. While there is no consensus explanation for this, but bigger solid state drives (SSD) often have faster write speeds than smaller drives. It could also be possible that a different vendor may be used by Apple for its 32GB chips. Check out the video below:

Additionally, the latest benchmark report by Cellular Insights also suggests that the iPhone 7 running on Qualcomm modem performs 30% faster than its Intel counterpart. Cellular Insights also explains that while all models perform pretty much the same in perfect conditions, it however changes when the signal is weaker. You can read the full report by Cellular Insights here.