I reduced the cache size of dnsmasq server. Or FQDN
Pretty sure these have nothing to do with each other. The PDU message is Wireshark telling you it's looking for multiple related packets, such as the sequence of packets making up an entire http response. It should have nothing to do with dnsmasq caching DNS entries
Or FQDN. Or no?
There's likely nothing "wrong" (or "evil") about that message. It's just wireshark collecting packets together that it believes are related into a "complete" protocol unit. Unless you've set the MTU lower on an interface than you "should" (or you have an application running that is unexpectedly sending jumbo packets over a "conventional" link), it's most likely just TCP working as expected.