It is a constant distance from the swing arm pivot to rear axle. The front sprocket is a bit offset(in front of) compared to the swing arm pivot, so this affects the chain slack as the swing arm moves up/down. Usually the chain gets tighter as the swing arm moves up(compressing the rear shock) and has the most slack when the swing arm moves down (extends the rear shock).
Location: Texas Hill Country (1hr NW of San Antonio)
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Pulsar2 is right on the money with his response.
BMW had a bike (G450X) for a while where the sprocket was right at the swingarm pivot so the chain tension remained constant throughout the entire up/down stroke. Interesting. But it must make other things difficult (like fitting an 'endless' chain!) or everyone would do it.