It all depends on the settings. Just as an example, let's say your page is 8x10. If you export normally, your PDF would also be 8x10, but since you want spreads, it would be 16x10. If you add the 1/8" bleed on all sides, you would add 1/4" to the length and width to get 16.25x10.25, which is what you seem to want.
What probably is happening is that you have crop marks on, which you probably do want to keep, because the printer will use them to either register when printing or align if placing into an imposed flat. The extra size you are getting is because the length and offset of the crop marks is being added to the size.
If you include the crops, you have to add the length of the crop mark, plus the offset. If your offset was set to .125, the crop would begin at the outer edge of the bleed image. At zero offset, the crop mark would begin at the trim, but you don't want that because if the cutter is off by a bit, you will see crop mark on the page. If the offset is .125 and the crop mark length is .25, you have to add .375 to each side, or .75 total, which would give you a PDF that was 16.75x10.75.