If your site uses SSD backed storage then size is directly related to speed. When storage is reduced, or increased, another critical set of attributes are also impacted- Input/Output Operations Per Second (IOPS), and, related to IOPS, is Burst Duration. The impact is linear from 100GiB upwards, with SSD storage actually slower than magnetic storage below this value.
The number of IOPS to expect at different sizes:
At 100GiB IOPS = 300
At 250GiB IOPS = 750
At 500GiB IOPS = 1500
The second aspect to this equation is Burst Duration. Storage can burst for periods of time, to 3000 IOPS. The duration of these bursts is limited by the size of the storage:
At 100GiB IOPS maximum duration of burst, in seconds = 2,000
At 250GiB IOPS maximum duration of burst, in seconds = 2,400
At 500GiB IOPS maximum duration of burst, in seconds = 3,600
Time to recover from burst use is slower on small capacities. A totally empty burst capacity results in refill times of:
At 100GiB burst refill, in seconds = 18,000
At 250GiB burst refill, in seconds = 7,200
At 500GiB burst refill, in seconds = 3,600
What is the take away for SSD backed storage?
Smaller storage sizes, will start to use burst capacity much sooner than a larger storage instance. Once used, smaller storage will take much longer to replenish burst capacity, and are bound by their default IOPS, as noted in the first table.
If you have a busy database, or the site makes very active use of file storage, a larger storage space may be the most cost effective performance boost possible. It may also be a very cost effective form of insurance against sudden and heavy traffic. Database binlogs can fill very quickly in certain situations. Having extra space for the database to cope with sudden growth, and, getting all the extra performance that the extra SSD storage size brings, is a win in every way for site stability and speed.