This is a nice looking fly, I really like the color combination. For someone who hasn't been tying spey flies very long, it is impressive.
To "thin out the body hackle", simply strip one side of the schlappen feather, which should be tied in by its butt.
To my eye, the tail needs to be just barel;y past the bend in length on a spey fly; therefore, I'd shorten the tail a bit.
I'd also lengthen the mallard wing slightly so that is as long as or a bit longer than the tag.
Small heads are produced by using only the bare minimum number of thread wraps to hold a material in, almost always only 3-5 turns of thread. Don't worry about the seeming lack of durability with so few thread wraps because when used properly, 3-5 turns of thread holds just as securely as 10 turns. And if you put cement on the wing and cheek butts before whip finishing, it reduces the head size quite a bit and cements the materials to each wrap of thread individually creating a very strong and durable fly.