It is not possible to change the disk size of the container:
C:\> vzctl set 101 --save --diskspace 100000000 Not enough disk space: only 79561136 blocks is free.
The container image file root.efd occupies much more space than the actual data inside of it:
C:\> vzquota stat 101 disk resource usage hardlimit path C: 1k-blocks 38079456 71859200 C:\vz\Private\101\root.efd
The root.efd file, on the other hand, occupies a large amount of space:
C:\> dir C:\vz\Private\101\root.efd ... 02/02/2016 02:31 AM 124,725,710,336 root.efd
The following warning is recorded inside
L=WARNING; R=87(135); C=0(0); M=135  Not enough disk space to shrink: CTID 101 disk C:
It is required to shrink the image file of the container, but the hardware node doesn't have enough space to perform this operation.
Increase the amount of free disk space to perform the shrink. The container can be shrunk either automatically next time or manually as described in the following article:
3972 Virtuozzo shrink options