No 
dblock is usually either 100KiB or 1MiB. If you have 4GiB dblock, it means you will look for deduplications on blocks larger than 4GiB? That requires volumes larger than 4GiB then…
True, if it really is dblock and not volume size, then it cannot be changed.
It is Duplicati doing the wrong thing here, buffering the entire request in memory.