According the paper 'Information propagation in the bitcoin network'. by Decker and Wattenhofer. For sizes larger than 20kB the cost can be said to be constant, whereas for small sizes there is a considerable overhead. Actually, I didn't get how small blocks take more time than messages whose size is greater than 20KB ? How delay taken by blocks with size greater than 20KB is almost constant? As shown in image below: .
The y axis of the plot shows the delay/size, that is, the number of seconds of delay produced by each kilobyte of data of the block.
Therefore, as blocks get bigger, the overall delay of the block also gets bigger. However, the delay per kilobyte of data approaches a constant value. This is produced because the overhead generated by the bitcoin protocol when sending blocks (anounce the block via an inv message, send getdata to retrieve content, etc.) is less significant as the block gets bigger.