Selecting 'Speed' gave me the following, which is closer to what I'd expect as 'normal' but still significantly reduced in size.
back to 'heatmap' yields the original result.
Also, as I was typing the above it occurred to me the measurement of the plan might have been incorrect, resulting in an out-of-scale situation causing this, and so I've just reconfirmed the measurement of the plan/scale is indeed correct.