It’s not completely clear, but I guess the authors make sure to keep the spatial size equal as described in 3.2:

[…]
To obtain an exponential increase of the receptive field,VGG-16 uses a max-pooling and downsampling layer after each block. However, this implementation would decrease the  resolution  of  feature  maps  exponentially.  Therefore,  to preserve  resolution  through  the  network,  we  remove  the max-pooling  and  downsampling  layers  and  use  dilated convolution in each block.
[…]