Why all Pendrives physical size are same

Why all pen drives physical size are the same is just like the same question, why mobile phones having the same sizes have differences in performance. In this case, we all know that a mobile phone's performance depends on its processor, not upon its size. The same case is applicable for pen drives as well as memory cards. This type of storage device's storage capacity does not depends upon its physical size but depends upon the technology it has been built.

Why all Pendrives physical size are same

Why all Pendrives physical size are same ?

In a very simple way let's consider two buildings A &B are built in the same size of the area and having the same hights. Now from the outside, you may think that both of the buildings have the same amount of inner space. Now consider that the walls of building A has thicker walls than building B. Now you can understand that building A should have less inner space than building B. Almost the same thing is also applicable to solid-state memory devices or pen drives and memory cards. These types of memory devices are made of transistors and logic gates. The more efficient the transistors are the more speed and storage you will receive from. And this is the reason the price also differs from one to another. 

The memory device having more storage capacity is more costly than lower storage capacity devices. You will see the labels like 'HC' 'XC' on different memory cards. The secure digital (SD) cards having storage capacity in between 32 Gigabytes are labeled as High Capacity (HC) or SDHC. The memory cards having more storage capacity labeled as Xtended Capacity (XC) or SDXC. And it is simply understandable that the SDXC is using higher technology than SDHC. 

How the both 2 GB and the 64 GB pendrive in the same physical size?

The chipset used in SDXC is called the NAND Flash memory chip. This type of chip is very cost-effective. It reduces the production cost of per-bit. And also increase the memory capacity in the storage unit. That's why you can notice that the memory card or pen drive having twice the storage capacity is less than the price of twice the price of the single one.
And the most important part is that this type of NAND flash memory is a non-volatile memory unit. That's why it can retain data after the power goes off. So that we can use it as our memory device.

why both 2GB and 64GB pen drive have the same physical size

Hope this information was quite helpful for you to understand why both 2GB and 64GB pen drive have the same physical size, and why all memory card's physical size is the same.