“He’s useless against their manipulations.”
(Season 11, Issue #7, Page 21 - Christos Gage and Rebekah Isaacs)
When Spike first brought home the then kittens (now cats) in Season 10, Xander was shown to be less than thrilled at the idea and a bit overwhelmed:
(Season 10, Issue #11, Page 22 - Christos Gage and Megan Levens)
Now, as we all know, some of those kittens ended up staying with them, and that’s because both Spike and Xander could not resist their cuteness:
(Season 10, Issue # 11, Pages 18 and 22 - Christos Gage and Megan Levens)
Spike is still able to keep his cool composure and go to work, but Xander totally just melts. This was the beginning of the manipulations.
In the first picture in this post (from Season 11, Issue #7), Spike warns that Xander feeds the cats too much. And we actually get shown this! In Season 10 we do see them eating quite a bit, but we only see Spike feeding them once over the course of numerous amounts of issues:
(Season 10, Issue #22, Page 13 - Christos Gage and Rebekah Isaacs)
In Season 11, however, Xander is show feeding the cats twice…
(Season 11, Issue #3, Page 3 and 23 - Christos Gage and Rebekah Isaacs)
Within the same issue, haha! XD
And they certainly have gotten a bit heavier since we were shown Spike feeding them in Season 10. The visual and written continuity of Spike and Xander’s journey as cat dads really makes that small comment of Spike’s to Willow all the better and more amusing.
It’s also nice that you can actually see the evidence for the fact that Spike is the stern, health conscious cat dad (look at his face when he gives them their food - he looks very concentrated and concerned that he gave them the proper amount) and Xander is the “you guys are too cute, here, eat all of the food!” cat dad (look at Xander just pouring all of that cat food all over the place without even looking down at how much he is giving them or anything - lol). And this actually fits in well with their personalities!
When it comes to Spike and people and things that he loves, he tends to be very protective and super conscientious of what they need within in his care. He often brought Drusilla food and prompted her to eat, and when he was looking after Dawn (and even afterwards, especially in the comics) he was always tried to keep her safe.
Xander, on the other hand, we have never really seen care for anything outside of his egg in Season 2′s Bad Eggs, and…well…he hard boiled his egg and then almost ate it, lol. He’s gotten quite a bit better than that, of course, but that kind of laid back approach to caring is still there.
All of this is why the “Spander Cats” background plot is one of the best things to come out of the Buffyverse and is really one of the highlights of the Season 10 and 11 comic seasons! <3