ram - Xen DomU with > 520GB memory

i am currently trying to assign 750GB Ram to a DOMU running on our Xen server. Even though the overview states that the DOMU has 750GB RAM, inside the Guest there are only around 520 GB available.

Here's the result from xl list:

Name                                        ID   Mem VCPUs      State   
Time(s)
Domain-0                                     0  8192     8     r-----  262366.1
guest1                                       6  8192     8     r-----  601597.5
guest2                                      20 64000    12     -b----  314883.1
guest3                                      21 32000    12     -b----   57891.6
guest4                                      24 64000    12     -b----  185059.3

guest5                                      27 64000    12     -b----  995675.0
relevant_guest                             32 750000    48     -b----     143.0

Here's the result of cat /proc/meminfo from inside the guest:

:~$ cat /proc/meminfo
MemTotal:       525887036 kB
MemFree:        520156652 kB
MemAvailable:   518429204 kB

Buffers:           41296 kB
Cached:           458620 kB
SwapCached:            0 kB
Active:           351672 kB
Inactive:         289840 kB
Active(anon):     142404 kB
Inactive(anon):    10552 kB
Active(file):     209268 kB
Inactive(file):   279288 kB
Unevictable:           0 kB

Mlocked:               0 kB
SwapTotal:      65533948 kB
SwapFree:       65533948 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:        141856 kB
Mapped:            96300 kB
Shmem:             11252 kB
Slab:              89588 kB
SReclaimable:      36052 kB

SUnreclaim:        53536 kB
KernelStack:        9408 kB
PageTables:        14504 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    328477464 kB
Committed_AS:     988804 kB
VmallocTotal:   34359738367 kB
VmallocUsed:           0 kB

VmallocChunk:          0 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
DirectMap4k:    768081920 kB
DirectMap2M:           0 kB

And finally, to put things into context, here's the result of xl info:

release                : 4.8.0-30-generic
version                : #32-Ubuntu SMP Fri Dec 2 03:43:27 UTC 2016
machine                : x86_64
nr_cpus                : 80
max_cpu_id             : 79
nr_nodes               : 4
cores_per_socket       : 10
threads_per_core       : 2
cpu_mhz                : 1895

hw_caps                : b7ebfbff:77fef3ff:2c100800:00000021:00000001:00003fbb:00000000:00000100
virt_caps              : hvm hvm_directio
total_memory           : 1048339
free_memory            : 44616
sharing_freed_memory   : 0
sharing_used_memory    : 0
outstanding_claims     : 0
free_cpus              : 0
xen_major              : 4
xen_minor              : 7

xen_extra              : .0
xen_version            : 4.7.0
xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          :
xen_commandline        : placeholder dom0_mem=8192M,max:8192M xsave=1 dom0_max_vcpus=8 dom0_vcpus_pin
cc_compiler            : gcc (Ubuntu 6.2.0-5ubuntu12) 6.2.0 20161005
cc_compile_by          : stefan.bader

cc_compile_domain      : canonical.com
cc_compile_date        : Fri Oct  7 19:25:16 UTC 2016
build_id               : af4763ee48b5462619bc61a6a248ae6b2dc75a79
xend_config_format     : 4

Any help would be appreciated. Thank you :-)