Why nucleus is the largest organelle in animal cell?

The nucleus must enclose the entire genetic complement of a cell

• and in molecular terms that is enormous. The human genome is

about 3 gigabases, which if it wasn't spooled up in chromatin and chromosomes would measure a few yards long. In addition, there must be room for all the proteins associated with DNA (structual proteins like histones, regulatory proteins like transcription factors, DNA polymerases, DNA repair proteins...) and of course enough void space for molecules to diffuse through and reach their targets. Thus, the nucleus must be comparatively massive. No other organelle seems to require such size and it might even be detrimental to many - the mitochondria for example with its reliance on establishing a proton gradient for ATP production would be far less efficient if it was the size of the nucleus. Imagine how many more protons would have to be pumped to set up a gradient if the mitochondria was that big! Smaller sizes are often better as they permit the creation of areas of high concentration of enzymes and molecules without the cell having to engage in massive synthesis.

It should be stressed that when cells do need bigger structures they can build them. Multicellular organisms often have specialized cell-types where other organelles are vastly larger than the nucleus. Adipocytes / fat cells in animals for example contain a single massive fat vesicle that dwarfs the nucleus. Similarly, the actin / myosin bundles in muscle cells are far larger than the nuclei of those cells.