Why is the shift key and enter key so large?

Human interface studies at DEC? No.

They are large because in 1950 you had to hammer those babies to move a few pounds of printing drum. The 'e' key is used more often than the shift key, if ergonomics was the issue then much of the keyboard would be different.

We never gave up the model of key shapes, which we can do at any time without leaving the basic keyboard layout. Apple did, reshaped the keyboard a bit, got rid of the old leftovers, but kept the basic layout. 

Backspace is another. The drum was spring loaded to move left to right as the user types, backspace had to work against the spring, and got the big hammer.  Tab got shrunk a bit, but tabs were always spring loaded in the easy direction.

Too many keys. I hardly use all those keys to the right of the letters, the number pad, and the function keys were always a huge waste, in my opinion. I would rather have a key bard with only the most commonly used, the basic alphabet, punctuation, numbers, and enter/shift,tb,backspace. I might keep the arrows, definitely dump the function keys, keep the space bar. Get rid of caps lock, the most horrendous key invented.  Control key is a left over from DEC days when you had to send a control character to the uart chip.