Why do I turn into super Mario instead of fire Mario when I touch a fire flower AS SMALL Mario in Super Mario Bros 1?

For a technical explanation, if we look at the disassembled code for Super Mario Bros, we can see that the routine for collecting fire flowers and super mushrooms is exactly the same.

HandlePowerUpCollision:
  jsr EraseEnemyObject    ;erase the power-up object
  lda #$06
  jsr SetupFloateyNumber  ;award 1000 points to player by default
  lda #Sfx_PowerUpGrab
  sta Square2SoundQueue   ;play the power-up sound
  lda PowerUpType         ;check power-up type
  cmp #$02
  bcc Shroom_Flower_PUp   ;if mushroom or fire flower, branch
  cmp #$03
  beq SetFor1Up           ;if 1-up mushroom, branch
  lda #$23                ;otherwise set star mario invincibility
  sta StarInvincibleTimer ;timer, and load the star mario music
  lda #StarPowerMusic     ;into the area music queue, then leave
  sta AreaMusicQueue
  rts

Shroom_Flower_PUp:
  lda PlayerStatus    ;if player status = small, branch
  beq UpToSuper
  cmp #$01            ;if player status not super, leave
  bne NoPUp
  ldx ObjectOffset    ;get enemy offset, not necessary
  lda #$02            ;set player status to fiery
  sta PlayerStatus
  jsr GetPlayerColors ;run sub to change colors of player
  ldx ObjectOffset    ;get enemy offset again, and again not necessary
  lda #$0c            ;set value to be used by subroutine tree (fiery)
  jmp UpToFiery       ;jump to set values accordingly

I would say this is intended behavior. If you somehow could get a super mushroom onscreen while you were Super Mario, he would turn into Firey Mario. Most likely, the rationale is to save space on the ROM.