why the size of image become large?

now I have a image of linux . the size is 1.5 M , I use the next command

$SOPC_KIT_NIOS2/bin/elf2flash --base=0x00000000 --end=0xffffff --reset=0x0 --inp

ut=vmlinux --output=ext_vmlinux.flash --boot=$SOPC_KIT_NIOS2/components/alter

a_nios2/boot_loader_cfi.srec

# Programming flash with the project

$SOPC_KIT_NIOS2/bin/nios2-flash-programmer --base=0x00000000 ext_vmlinux.flash

so i get ext_vmlinux.flash . but i find a problem . the size of ext_vmlinux.flash become very large . it have 3.5M .

so I want to know why . when I choose a flah . should we select 2M or 4M .

how to get the size of flash .

pls help me !