whole boilies have a certain size like 5mm or 8 or 10 etc. crushed boilies are smaller than those you can find in the shop. Size of incredients influence fish size. smaller pieces will atract smaller fish and bigger ones will call bigger fish. that doesn't mean you will get only big fish if you use biggest boilies or you get only small fish if you use the smaller ones. Think about it like this and i will give an example that may or may not be true but i use it to create a picture for you to better imagine how it is: lets say you try to catch grass carps and you want to make a pva or groundbait with "field grass" aroma boilies. You will have the option to use biggest boilies you find in shop 8 or 10mm ,forgot what was, or you can crush them. Using them crushed will atract fish between 1kg and 30kg with a 70% chance to be 1-10kg , 29% chance to be 10-20kg and 1% chance to be 20+ . That will mean you will have low chances to get a trophy, a bit more chance to get a decent size and very big chances to get small grass carps. That will translate into higher bite rate since smaller fish usualy bite more frequently. So using smaller fragments of the boilies (crushed) or the minimum size from the shop, will give you bite rate.
Using whole boilies and of bigger size, like 8mm or 10mm , will give you like 40% chance to get a 1-10kg, 30-35% chance to get 10-20kg fish and 30-25% chance to get over 20kg. so you have a lot more chances to get a big fish. the problem is that bigger fish bite more rare since they are more sensitive to conditions(weather, leader/line visibility,hook size,etc) so those percents I mentioned are from a total of maybe 10-15 fish per hour (1 game day) while the examples i gave for smaller(or crushed) boilies are from a catchrate of over 15 fish per hour. Keep in mind that every single number and percent I mentioned is fictional and with the purpose of explaining the best i can, this are not true data , it's just numbers for you to create a picture how smal vs big boilies work. bigger = bigger fish, much slower catch rate, smaller(or crushed)=more frequent bites, most of the fish small or average. You will still be able to get big fish on smaller boilies but only if you are VERY lucky or you can still get small fish if you get bigger boilies, but you gain more access to the bigger ones . And it's about same deal with hook sizes in case you wonder. i hope you got some clarity if not, sorry for my explanation , i tried :P .