White space is created when mail is deleted in your store. Mail is only finally deleted from the store when everyone who had a 'copy' of it (actually a pointer to the item) 'deletes' it; and when your Deleted Item Retention timer expires the item finally.
White space can be used again by the store to save new messages. So, if the average amount of new mail being created is higher than the average amount of messages being deleted, the trend will be for all white space that is created to be immediately reused, and still the size of the file will need to grow to store the new messages.
Your question really is: is all my growth caused by legitimate message traffic, or do I have some kind of mail loop that is consuming space? I would think that some kind of mail loop would confume space much faster than 1GB per month, therefore it's probably down to the accumulation of ordinary email.
You don't say how many users you have, nor what kind of users they are (light, medium, heavy users of email). So it's hard to make a judgement from here.