I was wondering what the difference is between the regular loop tip an large loop tip an what each one is used on an why does it have to with the weight of the rod etc thanks

The large tip-tops on fly rods are only necessary when the line-to-leader connection or the line-to-backing connection is large. There is plenty of information on line about how to make these connections smooth and smaller than the belly of the fly line you are using. Quality fly rod blank manufacturers go to great length to shave fractions of an ounce off their blanks, especially leveraged weight at the tip of the rod. We pay dearly for lighter blanks. On the other hand, if your fly casting involves only casts of 35 feet or less the size and weight of the tip-top won't make much difference.

As mentioned above, the larger loops are to facilitate passage of knots and loop-to-loop connections. The large loop is rarely used on lighter weight fly rods.

Other than shooting head loop-to-loop knots, knots of any sort - i.e. connecting line to leader and knots used to make hand made tapered leaders have no business being inside your guide train while fishing.
You'd be hard pressed to find a tip-top loop that can't pass a nail or barrel knot while stringing your rod.
However, you should be concerned with the ability of running guides and tip-top to pass a running line snarl with a hot fish on. So your tip-top ring should be same size as the smallest running guide - or slightly larger - not smaller.
Herb

I agree with Herb that the tip top should equal to or be greater than the smallest running guide. I always go a little bigger on the tip top. The knot that may actually pass through your guides with a fish on will be your fly line to backing knot. I think the best knot for this is the FG knot with a little flexible super glue on it to help with the transitions.