Does your iPhone 7 Plus take longer to charge than 6 Plus? Seems mine does.
The 7 Plus has a larger capacity battery.
Your 6 Plus battery capacity has likely faded due to use and aging. Lower capacity due to unusable portion means less time is required to charge it.
You should always use the included 5W compact charger when possible. Less heat during charging is better for battery health, even if it means the charge takes longer.