In standard German (of Germany and Austria) we use the spelling difference between ss and ß to signal whether the preceding vowel is short or long. A short vowel is followed by ss, a long vowel is followed by ß.
In Swiss standard German (N.B. I really mean Swiss standard German here, and not Swiss German), there is no ß letter, and words which would include it in Germany and Austria are always spelt with ss.
Is this just an orthographic fact, or does it also mean that the vowel before ss is always short? In other words, in Swiss standard German, would you read Strasse with a long or short a?
ßare not used to indicate whether the preceding vowel is short or long (even though you can guess from that). Rather, it is the other way: the length of the preceding vowel decides whether you need to use
ß, which is an important difference and already answers your question. In "de-DE" German, the decision is
short vowel => ss, long vowel => ß. As there is no ß in "de-CH" German, the decision is
short vowel => ss, long vowel => ss. The problem is the other direction. Does Massen refer to Massen or Maßen?