I'll second that. I always paid close attention, and I was pretty sure that it didn't look like I was getting quite 1 1/2 times the amount.

To use the Grand Dragon for example - because I've fought hundreds of them over the years and the numbers that appear at the end of every battle are permanently imprinted into my brain - they give a four character party a base amount of a tweensy bit over 8800 experience per character, and I've counted many times that party members with the Level Up ability equipped receive not quite 1200 experience in comparison. That works out to be closer to 33% than 50%.