I would separate the notion that a phonetic sound in かな represents a word from the fact that かな is used to write the syllabic sounds used in Japanese. However, I think you mean to ask the question of whether the combination phonetic sounds generated by normal かな + small かな, e.g. きゃ、しゅ and so on, are considered a single syllabic mora. That answer would be yes, despite the fact that it takes 2 かな to write.
For extra background, a mora is a unit of syllabic weight, and is usually characterized by what feels like a syllable in pronunciation, unless the syllable contains long stress, in which case the value of the syllable is two.
So, in the case of きょ and such other sounds, this is considered one mora, because the pronunciation is a quick one, as the sound that is made is a combination of the sounds from both かな but is pronounced with a fluid non-stressed tone.
Finally, to answer your question directly: This is not like a contraction in English, because each かな cannot be considered a word on its own. This is closer to combination consonant sounds, as ajsmart answered previously, but is also not quite the same because of vowel sounds.