The present disclosure relates to image forming apparatuses and image forming methods and particularly relates to a booklet printing technique. 
Various image forming apparatuses with a booklet printing function have been proposed in the past.  The booklet printing refers to a technique of reading one side or both sides of each of a plurality of original documents and producing prints in which a set of images formed from the read original documents are arranged on both sides of recording paper sheets of a size equal to two original documents laid side by side, with four pages per recording paper sheet, to match the page order of the original documents during reading with the page order of the resultant prints laid one on another in booklet form.  The recording paper sheets thus printed in booklet page order are discharged as a booklet in which a stack of them has been center-folded by a post-processing unit. 
As an image forming apparatus capable of performing booklet printing, an image forming apparatus is known in which when, during printing for original documents of mixed document sizes, the size of an image data set generated from one of the read original documents is greater than an image forming area of a recording paper sheet, the scale of the image data set is changed by calculating an appropriate reduction ratio to fit the image data set in the image forming area, which is one half of a printable area of one side of the recording paper sheet. 
In another known image forming apparatus, when the size of an image data set of a read original document is greater than an image forming area, the image data set is divided into right and left halves and the divided halves are allocated on recording paper sheets to form a pair of facing pages in a finished booklet.