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There is no backlog because there is no queue. Reviewing is a free-market system. It's up to the individual reviewer what they review. A writer is a shop owner seeking customers (readers/reviewers). Reviews are the evaluation forms customers fill out about your business.
Length matters when reviewers short on time or inclination want to do a review: they pick shorter pieces. Thus, longer items are less likely to be reviewed.
No, memberships don't matter for getting a review on a particular piece. When you have more space, you can have more items in the pool, but then you're also competing against yourself.
A membership is something you get for your own convenience: better tools for giving reviews, better tools for custom-fitting access to your items, better storage options for items, less ads, access to item statistics, things like that.
What does make a difference in getting reviewed is using the tools you have to increase the visibility of your item and its attractiveness to reviewers. The things in the 101 entry I linked earlier.