Not inconsiderable
Definition 1 of 1
Pro Tip 1/3Double Negative
Understand that 'not inconsiderable' is a double negative, which means it's actually positive. 
The city's park covers a not inconsiderable space, great for jogging.
Pro Tip 2/3Understated Emphasis
It subtly emphasizes that something is larger or more important than it may seem. 
Her contribution was not inconsiderable, impacting the project greatly.
Pro Tip 3/3Formal Context
It's more commonly used in formal writing or speech. 
The not inconsiderable efforts of our team led to success.