I have installed volumio-2.348-2017-12-22-pi on my old raspi B+. After first boot (and next too) overlay partition is not resized.
root@volumio:~# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mmcblk0p2 2.2G 625M 1.5G 30% /imgpart
/dev/loop0 277M 277M 0 100% /static
overlay 262M 188M 54M 78% /
devtmpfs 478M 0 478M 0% /dev
tmpfs 489M 4.0K 489M 1% /dev/shm
tmpfs 489M 17M 472M 4% /run
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 489M 0 489M 0% /sys/fs/cgroup
tmpfs 489M 32K 489M 1% /tmp
tmpfs 489M 0 489M 0% /var/spool/cups
tmpfs 20M 1.6M 19M 8% /var/log
tmpfs 489M 0 489M 0% /var/spool/cups/tmp
/dev/mmcblk0p1 61M 35M 26M 58% /boot
tmpfs 98M 0 98M 0% /run/user/0
SD card is 8GiB and parted reporting this
root@volumio:~# parted -s /dev/mmcblk0 unit MB print free
Model: SD SL08G (sd/mmc)
Disk /dev/mmcblk0: 7948MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
0.00MB 0.00MB 0.00MB Free Space
1 0.00MB 64.0MB 64.0MB primary fat32 boot, lba
2 64.0MB 2500MB 2436MB primary ext4
3 2500MB 7948MB 5448MB primary ext4
7948MB 7948MB 0.21MB Free Space