Comment # 12 on bug 922308 from Stefan Hundhammer

"root_base" does not occur directly in the control.xml file; it is a calculated
value:

StorageProposal.rb:228... :

        stmp = ProductFeatures.GetStringFeature(
          "partitioning",
          "root_base_size"
        )
        Ops.set(
          @cfg_xml,
          "root_base",
          Ops.divide(Storage.ClassicStringToByte(stmp), 1024 * 1024)
        )
        if Ops.less_or_equal(Ops.get_integer(@cfg_xml, "root_base", 0), 0)
          Ops.set(@cfg_xml, "root_base", 3 * 1024)
        end

So it's 3*root_base_size .

For Tumbleweed, this would be 3 * 5 GB = 15 GB.
For SLE-12,     this would be 3 * 3 GB =  9 GB.

The storage target maps from comment #7 and comment #9 report:
    No LVM: "size_k": 10490445  (~10 GB)  (separate /home partition)
    LVM:    "size_k": 19435520  (~19 GB)  (/home directly on Btrfs / on LVM)

The values are so much different because in the first (default) proposal with
just Btrfs, a separate /home partition was proposed (which takes the rest of
the disk space), while during the change to LVM, the user unselected the
"propose separate home partition" checkbox with just one large Btrfs on LVM
spanning the entire disk (see macro_inst_initial.ycp and newly appearing
subvolume "home" in comment #7).

The other calculated values in the proposal are identical before and after the
change to LVM:

2015-03-13 09:15:18 <1> 10.0.2.15(2721) [Ruby] modules/StorageProposal.rb:327
GetControlCfg after :
$[
    "btrfs_increase_percentage":100,
    "home":true,
    "home_limit":14336.0,
    "home_max":51200.0,
    "prop_lvm":false,
    "prop_snapshots":true,
    "root_base":10240.0,
    "root_max":40960.0,
    "root_percent":40,
    "suspend":false,
    "vm_keep_unpartitioned_region":false,
    "vm_want":30720.0
]

2015-03-13 09:15:46 <1> 10.0.2.15(2721) [Ruby] modules/StorageProposal.rb:327
GetControlCfg after :
$[
    "btrfs_increase_percentage":100,
    "home":true,
    "home_limit":14336.0,
    "home_max":51200.0,
    "prop_lvm":false,
    "prop_snapshots":true,
    "root_base":10240.0,
    "root_max":40960.0,
    "root_percent":40,
    "suspend":false,
    "vm_keep_unpartitioned_region":false,
    "vm_want":30720.0
]