Resset567 wrote: Red or pink leaves are normal on peony sprouts. Some varieties keep a reddish cast to their foliage all season. See photos below taken a few minutes ago, the young plant with very red shoots and the pair of mature plants, one with red shading on the foliage and the all-green variety behind it.
First: Are both plants the same variety? If not, you may have two double pinks that sprout and grow at different rates.
Second: Do both enjoy the same growing conditions? Same amount of sun, same drainage, same amount of impact from the cedars? Like WeeNel mentions, peonies grow much better with good sunlight. So, if the smaller one is in a shady site, it may have struggled in prior years and this spring it just doesn't have much energy to develop its new foliage. If that's the case, I'd move it now rather than lose it.
Third: Is this the first year you've experience this? I ask because peonies are fussy about the depth at which they are planted. They like the eyes/sprouting points to be 1.5-2" below the soil level. Plant them deeper and while they'll often live, they won't thrive and will be smaller, less robust and often won't bloom.
If these three questions don't provide clues, I'd take a leaf from the smaller plant to a good garden center to have them check for botrytis which is a fungus that tends to shrivel leaves and is particularly prevalent during wet springs.