I have a thread on this. The 3885W set will have slightly more tension overall than the S42W set, and will be more even from string to string. Since your Kay is a 42 inch mensure, it will have a tad more tension than the "standard" 106mm, 41 3/4 inches that the tension figures are based on.
I'm surprised you think they sound "thinner," as overall, there has to be a tad more mass in each string in order to have the greater tension, and when I changed from a S39W to a 3885.5W I noticed more body to the tone as well as the very slight increase in tension.
Then again, brand new strings will all sound artificially bright until they settle in after a couple of months. Give them some time. I think you will come to really enjoy them.
Here's my thread on all the Spiro sets. Scroll to the bottom where I had to fix some typos. All The Different Spiros Sets