Well, as one of the largest hiders of micros in Iowa, allow me to put my two cents in on this topic! I don't think the issue is so much one of size, as it is of proportion - how does the size of the cache fits its surroundings? There will be a column in an upcoming IGO newsletter called "What's Reasonable?" (modesty prevents me from naming the author ) which addresses exactly this controversy.
Consider some of these situations, with my opinion:
1) Micro on ground in woods in half-acre of fallen leaves, bad.
2) Micro on tree in woods (thinking of Mad, Mad World of Micros in Marion), no problem.
3) Ammo can in plain sight in woods, bad.
4) Ammo can hidden in fallen tree, better.
5) Ammo can hidden in place 10 feet from what appears to be logical hiding place, great!
Everything in life grows and evolves - things that stay the same we label as "dead." For some, caching is a long walk in the woods. Others say there is no challenge, no "game" or "sport" to that. I've been to a cache series in a park near Peoria, IL, where every single can is highly visible. What's that called? Hiking! Actually, I was a hiker before becoming an "off-road" hiker, i.e. a geocacher. I think a good thing to remember is that no one "owns" the sport. It's nice if you were here in 2001 or 2002, but does it make you better, or more qualified to judge another? Innovation is always challenged and criticized, but I wander...back to topic,,,it's really a question of striking a balance between a challenge and a feeling that it can be found with a minimum of frustration. If cachers feel it's a fair contest, then success has been achieved....And, then there's Hanna Park Micro...arrrrrrrgh!