Why is the US one of the few countries that can't keep a consistent design across their bills and still does not have a way for blind people to identify which note they have. They are all the same size and have no embossed markings.
The redesigns were mainly about preventing counterfeiting. That's also the reason for continuing to change them periodically. In some places outside the US money shops and banks won't exchange the old style notes after a certain time. Changing the design resets counterfeit operations back to square one. Then they added the embedded strip. Color doubles or quadruples the number of printing plates and steps needed and truly large (or state-sponsored) operations may have to start over when that happens.
However someone in Congress got a bee in their bonnet about the changes and passed a law that prohibits Treasury from redesigning the $1 bill so it will continue to remain disjointed. That irks me tremendously. There never were any plans to redesign the $2 which also irks me.
As for bill size Treasury/Fed has this philosophy not to stray too far for the appearance of stability. The US doesn't demonetize currency unlike many governments so your grandma can redeem an ancient $20 for a modern one without question. The cotton rag paper, identical size, etc is part of that stability. Also why the color changes were so subtle. The secondary reason is the huge chaos it would create as millions of bill acceptors will not or even cannot take a firmware update and would need complete replacement to handle a new size.
You might not _agree_ with the reasons. They may not be _good_ reasons. But stability/tradition, Congress being idiots, and expense are the reasons the bills don't change much (or at all for $1/$2).