Have you checked on another application to see the reporting size? I would use GPU-Z or similar program and see what it reports.
I don't know about `gpu-z` on Ubuntu, but I did try a few things:
sudo lspci -v -s 01:00.0
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Device 67df (rev e7) (prog-if 00 [VGA controller])
Subsystem: ASUSTeK Computer Inc. Device 0521
Flags: bus master, fast devsel, latency 0, IRQ 16
Memory at c0000000 (64-bit, prefetchable) [size=256M]
Memory at d0000000 (64-bit, prefetchable) [size=2M]
I/O ports at e000 [size=256]
Memory at dfe00000 (32-bit, non-prefetchable) [size=256K]
Expansion ROM at 000c0000 [disabled] [size=128K]
Capabilities:  Vendor Specific Information: Len=08 <?>
Capabilities:  Power Management version 3
Capabilities:  Express Legacy Endpoint, MSI 00
Capabilities: [a0] MSI: Enable- Count=1/1 Maskable- 64bit+
Capabilities:  Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
Capabilities:  Advanced Error Reporting
Capabilities:  #15
Capabilities:  #19
Capabilities: [2b0] Address Translation Service (ATS)
Capabilities: [2c0] #13
Capabilities: [2d0] #1b
Capabilities:  Latency Tolerance Reporting
Capabilities:  Alternative Routing-ID Interpretation (ARI)
Capabilities:  L1 PM Substates
Kernel driver in use: amdgpu
Kernel modules: amdgpu
`sudo lshw -class display` reports similar information, except in hex.
sudo dmesg | grep drm
[ 9.252075] [drm] amdgpu: 8192M of VRAM memory ready
[ 9.252075] [drm] amdgpu: 8192M of GTT memory ready.
This post describes a similar problem:
But of course in his case, he's only getting use out of 2GB while I'm getting 6GB.