Patch management for ESXi is very different compared to traditional operating system patches, where incremental updates are made to the base operating system and thus increasing the disk footprint for each patch update. For the ESXi hypervisor, when a patch is applied, the entire ESXi image also known as an Image Profile is replaced. This means that each time you patch or upgrade your ESXi host, you are not adding on top of the original installation size.

As part of the ESXi architecture, there are two independent boot bank partitions that are used to store the ESXi Image Profile. This is primarily used as a fail-safe mechanism for rollback.

Here is a diagram showing what the ESXi boot banks would look like before and after applying a patch (pertains to both updates and upgrades)

 --> The rest is to be read on the blog : http://blogs.vmware.com/vsphere/2013/04/understanding-esxi-patches-size-patch-bundles.html 

And dont forget to patch your ESXi, it is fast, easy and more secure!