Baoquan He [off-list ref] writes:

Is there a reason hyperV need boot with small memory, then enlarge it
with huge memory? Since it's a real case in hyperV, I guess there must
be reason, I am just curious.

It doesn't really *need* to but this can be utilized in e.g. 'hot
standby' schemes I believe. Also, it may be enough if the administrator
is just trying to e.g. double the size of RAM but the VM is already
under memory pressure. I wouldn't say that these cases are common but
afair bugs like 'I tried adding more memory to my VM and it just OOMed'
were reported in the past.

-- 
Vitaly