I am surprised after reading this thread that "Mr White Leghorn" Dan Honour has not posted on this thread. He has a strain of White Leghorn's that he raises that can be traced back to pre-WW2. His and other exhibition White Leghorns are much larger than the hatchery variety. Amazing tails on those birds.
IF you want to know what a Leghorn crossed with anything else all you have to do is look at almost every hatchery variety of every other breed. Cherry Eggers, Red Sex Links etc are all hybrids of White Leghorns and Rhode Island and New Hampshire Reds........Thats why their eggs are not a very dark tint of brown. Leghorns were cross bred into other hatchery stock so that more eggs would be laid and there fore more chicks could be hatched to be sold.
White Leghorns are exceptional chickens in their ability to produce eggs but they do not produce the largest white eggs nor do they have a large meat carcass either.
If you do want the biggest white egg then you need a variety of Minorca, preferably White or the more common Black. Most of their eggs will grade out as XL and usually a chalky white as opposed to a glossy white like those of the White Leghorn. Carcass size is much larger though, a good full grown Minorca cock will weigh 10 lbs and fully developed hen will weigh over 7 lbs. They are very large birds and are extraordinary. Saw a nice big female at the show I went to today, she was enormous! There were WF Black Spanish and White Leghorns on each side of her and she was twice the size of even the Spanish males......