There are several answers here. Which is the most palatable? 
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Initially, poster/cover art size was low resolution images. Users complained at the lack of quality. Engineering re-evaluated and agreed. It was increased.
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The amount of space used is obviously in direct proportion to the number of items indexed. 50,000 movies will consume about the same metadata space as 50,000 songs. Which is more likely to occur?
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Linux distros make the root partition very small if a
/homepartition is selected during installation. This creates the pressure-point on available disk space.
There is an easy way to work with whatever the distribution is doing. The most common is to relocate the metadata to the /home partition.
For secondary reference: I have about 20,000 total items (movies, episodes, and music) indexed. I am using just about 35GB for the entire metadata. The actual media itself consumes some 48 TB
