Maybe
You'll need to work with the DM.
You've quoted the relevant part of the spell description:
As the image changes location, you can alter its appearance so that its movements appear natural for the image.
To the extent it's altering its appearance for a "fog or darkness effect" to expand from 10x10x10 to 15x15x15, or contract to 5x5x5, then that's a reasonable interpretation of the spell. Or at least, a not unreasonable one. Although, the spell doesn't say the illusion can get bigger or smaller.
However, silent image, like all illusion spells, is going to require working with the DM to determine exactly what happens. A reasonable DM may well rule either way.
What would influence a ruling one way or another? Well, for one, you've created an "image of an object, a creature, or some other visible phenomenon". You can make the subsequent movements appear "natural", for that thing, whatever the illusion is.
Another factor that might influence a DM is the "cheese factor". Are you attempting to exploit the spell in some strange way to gain a benefit far beyond something a first level spell would do? A DM is likely to be more restrictive.
Yet another factor is that the spell description makes no mention of the illusion changing size. A DM can easily say no, based solely on that. However, the DM may allow it.
Finally, different DMs approach illusions in different ways. You're best off working with the DM to determine how illusions work in your specific game. Some DMs take a very liberal approach and others don't. Both are okay.
As a DM, I would be open to the idea of the illusion changing size, but I would want to understand what you're trying to accomplish. I would be thinking about how the spell is worded, the cheese factor, precedent for future applications of the spell and other illusions, and how to make sure that the resulting ruling doesn't somehow make the game less fun. To me, the RAW is important, but that's where the game starts, not where it ends.
