To put a finer point on it, the limit on cell size is closer to the definitional difference between what makes something a "plant" vs an "animal" than almost anything else. Plants, due to their stable tissue structures, don't need to worry about their cell size, and therefore don't need to worry about unbounded DNA growth. Because of this, they have evolved to take advantage of having "more" DNA. Yes, polyploid plants are generally healthier, because plants expect polyploidy. But even without polyploidy, plants have large amounts of "specialized-use" genetic material, libraries of specific responses (e.g. immune responses) to specific problems (e.g. parasites) that may have only existed millions of years ago. They can just keep all these one-off solutions around, evolving to protect themselves against one more thing at a time.
(This is also why plants tend to be full of weird organic molecules like terpenes, which are beneficial to nearly every animal alive today: those chemicals probably protected them from something at some point, and the genes responsible for making them have had no reason to be shut off since then.)
But animals, with a finite limit on gene-base size, must solve problems differently. Animals evolve along different lines—polyploid mutations usually result in non-viable offspring for animal species, because animals frequently have mutations that assume diploidy, with complex machinery to favor better vs. worse versions of each diploid gene using silencing or epigenetic methylation. And more generally, animals evolve solutions to whole classes of problems that have compact genetic representations. We don't have a million custom immune responses; instead, we have xenograft rejection genes, T-cells, livers and kidneys, saliva and stomach acid, the sensation of itching, skin that sheds and skin oil that clears pores, and put together, those mechanisms handle 99% of cases just fine.
And often we don't need any of those; the primary "solution" that animals have to almost every problem, is just the ability to move away from the source of the problem. If there's a predator, you can just run away. You don't need to know what the thing is, to run away from it; you just run away! Universal solution! To do that, we need a lot of things: not just limbs, but sensory organs, to know where we are and where the predator is; and nerves, to link the two; and even some sort of instinct for what predators exist, or memory to record sense-fear associations. But all those genes still take up far less space than immune proteases individualized for every bacterium and fungus on earth.
(This is also why plants tend to be full of weird organic molecules like terpenes, which are beneficial to nearly every animal alive today: those chemicals probably protected them from something at some point, and the genes responsible for making them have had no reason to be shut off since then.)
But animals, with a finite limit on gene-base size, must solve problems differently. Animals evolve along different lines—polyploid mutations usually result in non-viable offspring for animal species, because animals frequently have mutations that assume diploidy, with complex machinery to favor better vs. worse versions of each diploid gene using silencing or epigenetic methylation. And more generally, animals evolve solutions to whole classes of problems that have compact genetic representations. We don't have a million custom immune responses; instead, we have xenograft rejection genes, T-cells, livers and kidneys, saliva and stomach acid, the sensation of itching, skin that sheds and skin oil that clears pores, and put together, those mechanisms handle 99% of cases just fine.
And often we don't need any of those; the primary "solution" that animals have to almost every problem, is just the ability to move away from the source of the problem. If there's a predator, you can just run away. You don't need to know what the thing is, to run away from it; you just run away! Universal solution! To do that, we need a lot of things: not just limbs, but sensory organs, to know where we are and where the predator is; and nerves, to link the two; and even some sort of instinct for what predators exist, or memory to record sense-fear associations. But all those genes still take up far less space than immune proteases individualized for every bacterium and fungus on earth.