The way I’ve had it explained to me, plants as a general rule have a much, much larger genome than animals. The reason is simple. When an animal runs into a problem like not enough water, it can just get up and move. Plants are rooted where they are, prey to whatever comes along. They have to develop an arsenal of genes to deal with different situations, whether that’s drought resistance or producing various toxins so that animals don’t make a meal of them. It’s not like animals don’t do this to some degree - the immune system is incredibly elaborate - but not as much.