This is a question that is well answered by looking at an online Latin dictionary.
The noun decimatio is listed as meaning "the taking of a tenth", including the punishment you refer to.
This noun is derived from the verb decimare, "to take the tenth" or "to decimate".
See the linked entries for more details.
This verb appears to come from the ordinal decimus, "tenth".
The link to this and even decem ("ten") is so clear that the Romans are constantly reminded of the relation to this number.
In English this link has deteriorated, as the word no longer immediately reminds us of the words "ten" or "tenth".
Therefore it is not all that surprising that the linked dictionary entries of decimatio and decimare only mention things that are directly related to a tenth.
However, it should be born in mind that the people composing a dictionary might have been biased or even misguided by etymology.
It's sometimes hard to say what a text means exactly if there is nothing else about the matter than that text.
Perhaps there was indeed an instance where decimare meant "to reduce by a fraction" where the fraction was not 1/10, but due to lack of clear hints it was analyzed as meaning 1/10 anyway.
To figure out whether this has happened would require a far more detailed study.
While mistakes can and do happen, major misunderstandings are rare.
Generalized meanings are not rare in Latin and the dictionary I linked to offer mentions them, so it is telling that no such mention is found in these specific entries.
So, a more careful answer might be:
There appears to be no evidence that the word decimatio was used for other reductions than by one tenth.
Such use would not be unreasonable, as such generalizations are commonplace in language.
The generalization with descendants of decimatio is pretty common in other languages, but it does not imply that it must have also occurred within classical Latin.
(I assume that by Romans you mean the people living in republic or imperial Rome, which is roughly the same as speakers of classical Latin.)
What I have seen so far strongly points towards decimatio referring strictly to one tenth in classical Latin, but this is of course subject to change if I see more.