While I'm sure its larger than normal, its not actually the size of your arm is it? LOL
OK so this is the deal with fiber. When you eat it, a lot of its goes through your system undigested. As it does and it passes through your bowels, it is scraping against the bowel walls which is lined with a substance called bile and the fiber will scrape away the bile and take it with it on the way out. So a little fiber can produce a big poop because the fiber is taking other stuff along with it for the ride.
This is why eating a high fiber diet is good for cholesterol - because a primary component of bile is cholesterol. So once the fiber takes the bile (and its cholesterol) out the exit, your body needs to produce more bile to line your intestines to help it with digestion. Because the body usually can't make enough cholesterol on its own to do this (yes, the body makes cholesterol) it will go into your blood stream and take cholesterol from your arteries and veins and move it to your bowels to help make up the bile. So eat more fiber, remove the bile, body takes cholesterol from your blood system to make new bile and repeat and that is why eating a high fiber diet is good for your cholesterol.
Metamucil is very good at this and why doctors also recommend it for people with high cholesterol.
Give it time and you will have "normal" poops but don't necessarily think normal means poops like you used to have - you've changed your fiber levels so your poop will also change. Think more along the lines of "new normal".