It’s better to say “small packets such as NTP”. Unauthenticated NTP packets are 48 bytes, vs. 64 bytes for ping’s default ICMP echo requests. Having a busy NTP server port-forwarded behind a NAT is more challenging than most NAT scenarios due to the large number of “connections” (UDP is connectionless, but that’s nit picking). Most TCP and UDP packets are substantially larger, and there are many packets in bursts going through the same NAT mapping, while public NTP servers requires the NAT to set up mappings that are typically used for just one extra-small packet in each direction before the mapping times out (after 30s or so). So the amount of NAT work setting up and tearing down mappings for each kilobit of NTP server throughput is much higher than other types of traffic.