Simon,
What you're seeing is expected. The AGSPictureMarkerSymbol uses an image to render the symbol. The size of the symbol depends on the size of the image. So if the image is 6 pixels x 6 pixels, that's how big it'll be displayed on screen. Now, you can override the size property of the symbol to make it larger, but that would force the image to be scaled possibly making them fuzzy.
When you pinch to zoom in, the feature layer waits till the gesture is over to re-render its content, and in the meanwhile, it just re-scales the content on screen. It does this because you could potentially have a lot of features in your layer, and re-rendering it while the gesture is in progress would be very CPU intensive and might deteriorate the responsiveness of the map. Thus, the symbol might temporarily appear larger as it is re-scaled while the gesture is in progress. And yes, this behavior is specifically tied to the "static" mode.
The "dynamic" mode is more suitable to graphics layer (not feature layers) which typically have only a few graphics. In this mode, the content is drawn using the GPU (not CPU as with static mode). The GPU fares much better at re-rendering content on-the-fly while the gesture is in progress. So you would not see picture marker symbols temporarily appear fuzzy, they would always be their true size.
But in both the cases, the picture marker symbol is drawn based on the size of the image (unless overriden in code)
