Resource "Mailbox size" is set to Unlimited in reseller's subscription properties, but the maximum mailbox size is set to different value, for example, to 100MB.
Subscriber's (reseller's) mailbox size limit is less than Unlimited, it is set to 100MB in a service plan. Thus, even if subscription is customized by setting Unlimited mailbox size, the limit will be still taken from the service plan properties.
In order to increase the mailbox size for a subscription, increase the limit for the service plan that subscription is subscribed to:
Service Plans > reseller_plan_name > Resources > Mailbox size > Update and Sync
and for reseller's plan that subscription owner is subscribed to:
Service Plans > Reseller Plans > reseller_plan_name > Resources > Mailbox size > Update and Sync