Just to clarify. "Size matters uncut" keeps the original stats. of a unit.
You can not break down an original unit into smaller units. Whom, I assume if you could would be weaker.
But you can merge 2 units into 1 unit. Again I assume gives additional strength?
Does this merging, also affect the movement ability of units? More units take more space so movement is slower, or less may be faster.
Size Matters Uncut doesn't change the ability to split or merge nor any of the consequences of doing so.
Mathematically speaking, on Size Matters standard, units with a 15 total SM cat rating (Their SM Cat Rating is the sum total of all 3 SM categories, Size/Group/Quality, values they possess) have normal base scores. However, if their total SM Cat rating starts off at less or more than 15 then they are modified when they come into the game accordingly.
Each SM Cat rating up a unit achieves adds +150% strength (and many other scores) onto the unit. Thus if I have a SM Cat rating of 15 and my unit has a 6 str then my unit starts with 6 str. However, if the same unit were established to start with a SM Cat rating because of a particularly high Size, Group Volume or Quality rating that isn't fully countered by the other two having deficiencies, then my unit would start with a str of 9. The same is true going in the opposite direction.
On Size Matters Uncut, the baseline is not 15, it's whatever the unit's particular total from its beginning combat classes starts at. Thus the unit above with a 16 total SM Cat rating would still start at 6 strength but if merged with 2 other units of the same type would gain another SM Cat so would thus be given a base of 9 str for the merged unit.
Merges require 3 units of the same type and creates one unit with a Group Volume 1 higher. Splitting creates 3 units of the same type, just a Group Volume category less. This works the same in both versions of the option. This, however, is not the only way units can have their SM Cat total adjusted. There are promos that allow units to increase their Quality rating as well. They inflict a heavy penalty for their use and create some very interesting choices as to if and when you want to use them... they are very powerful but will keep the unit from any further development for a long long time.
Movement is not affected, just as it wouldn't be if you marched an army of 9 units down the road or one it would be the same. Many values are affected though and some values only consider the Size and Group Volume and ignore Quality (for purposes of unit load volume for example.)
So in summary, Size Matters Uncut is exactly the same as Size Matters with the one exception that some units are balanced a bit differently in Size Matters. I personally feel this has given Size Matters core a bit more room to balance itself properly in light of not all units being able to merge or split but for traditionalist players who appreciated some of the core game balance settings and don't like some of the rebalance choices made in Size Matters core, Size Matters Uncut was invented.