AWH memory usage

GROMACS version: 2020.2
GROMACS modification: No

I have encountered some unexpected behavior with the AWH functionality. I expected its memory usage to be more or less close to “a few arrays with one element per sampling point”. However, the GROMACS memory requirements actually increase dramatically with increasing force constant, soon hitting the physical memory limit (after which, predictably, it does not do anything useful and can only be terminated by KILL).
The test setup has 3 dimensions, total sampling space is 7x8x8 nm.

9.8g 0.4g no pull/awh active, all other GROMACS functionality
18.1g 8.8g k=250 d=0.099 464k sampling points
26.7g 17.4g k=400 d=0.078 948k sampling points
33.7g 24.6g k=500 d=0.070 1312k sampling points
58.4g 30.7g k=800 d=0.055 2705k sampling points

In addition, the checkpoint (CPT) files are quite large (3-4 gigabytes each in these test cases) while the energy (EDR) files where the AWH data are stored are quite reasonable (about 100 megabytes).

Might something be wrong here?