golang，uint、uint32、uint64这三种类型的区别是什么？

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    var a uint
    var b uint32
    var c uint64

    fmt.Println("size of a: ", unsafe.Sizeof(a))
    fmt.Println("size of b: ", unsafe.Sizeof(b))
    fmt.Println("size of c: ", unsafe.Sizeof(c))
}