The difference mainly lies in these aspects:
Capacity: A 5000mAh power bank can provide around 1-2 smartphone charges, while a 20000mAh power bank can offer approximately 4-8 smartphone charges, given ideal conditions.
Appearance: A 20000mAh power bank is physically bulkier due to the need to store more electricity.
Recharge time: A 20000mAh power bank takes longer to recharge itself due to the larger battery capacity.
Price: 5000mAh power banks are usually cheaper than 20000mAh power banks since they have lower capacity. For example, the Anker 511 Power Bank (PowerCore Fusion 5K), a high-performance 5000mAh power bank that can charge an iPhone 13 Pro in 1.3 hours, costs only $29.99.