How large was the Smilodon?
Smilodon
The brain of "Smilodon" had sulcal patterns similar to modern cats, which suggests an increased complexity of the regions that control the sense of hearing, sight, and coordination of the limbs. Felid saber-tooths in general had relatively small eyes that were not as forward-facing as those of modern cats, which have good binocular vision to help them move in trees. "Smilodon" was likely an ambush predator that concealed itself in dense vegetation, as its limb proportions were similar to modern forest dwelling cats, and its short tail would not have helped it balance while running. Unlike its ancestor "Megantereon", which was at least partially scansorial and therefore able to climb trees, "Smilodon" was probably completely terrestrial due to its greater weight and lack of climbing adaptations. The heel bone of "Smilodon" was fairly long, which suggests it was a good jumper. Its well-developed flexor and extensor muscles in its forearms probably enabled it to pull down, and securely hold down, large prey. Analysis of the cross-sections of "S. fatalis" humeri indicated that they were strengthened by cortical thickening to such an extent that they would have been able to sustain greater loading than those of extant big cats, or of the extinct American lion. The thickening of "S. fatalis" femurs was within the range of extant felids. Its canines were fragile and could not have bitten into bone; due to the risk of breaking, these cats had to subdue and restrain their prey with their powerful forelimbs before they could use their canine teeth, and likely used quick slashing or stabbing bites rather than the slow, suffocating bites typically used by modern cats. On rare occasions, as evidenced by fossils, "Smilodon" was willing to risk biting into bone with its canines. This may have been focused more towards competition such as other "Smilodon" or potential threats such as other carnivores than on prey.
Debate continues as to how "Smilodon" killed its prey. Traditionally, the most popular theory is that the cat delivered a deep stabbing bite or open-jawed stabbing thrust to the throat, generally cutting through the jugular vein and/or the trachea and thus killing the prey very quickly. Another hypothesis suggests that "Smilodon" targeted the belly of its prey. This is disputed, as the curvature of their prey's belly would likely have prevented the cat from getting a good bite or stab. In regard to how "Smilodon" delivered its bite, the "canine shear-bite" hypothesis has been favored, where flexion of the neck and rotation of the skull assisted in biting the prey, but this may be mechanically impossible. The mandibular flanges may have helped resist bending forces when the mandible was pulled against the hide of a prey. The protruding incisors were arranged in an arch, and were used to hold the prey still and stabilize it while the canine bite was delivered. The contact surface between the canine crown and the gum was enlarged, which helped stabilize the tooth and helped the cat sense when the tooth had penetrated to its maximum extent. Since saber-toothed cats generally had a relatively large infraorbital foramen (opening) in the skull, which housed nerves associated with the whiskers, it has been suggested the improved senses would have helped the cats' precision when biting outside their field of vision, and thereby prevent breakage of the canines. The blade-like carnassial teeth were used to cut skin to access the meat, and the reduced molars suggest that they were less adapted for crushing bones than modern cats. As the food of modern cats enters the mouth through the side while cutting with the carnassials, not the front incisors between the canines, the animals do not need to gape widely, so the canines of "Smilodon" would likewise not have been a hindrance when feeding.
Machairodus
"Machairodus" was similar in size to a modern lion, standing about at the shoulder. The species once referred to "Machairodus coloradensis" was apparently significantly larger, about at the shoulder, according to skeletal and life reconstructions. If accurate, this would have made "M. coloradensis" one of the largest felids. However "M. coloradensis", along with "M. kurteni" and "M. giganteus", has been reassigned to the genus "Amphimachairodus" and by extent, the Homotherini based on both morphology and its time of appearance in North America. "Nimravides catacopsis" also has been separated from "Machairodus", as the two are too different in regards to autapomorphies to be synonymous. However, the species "Machairodus horribilis", which stands as the largest known species of the genus, is comparable in size to the much later "Smilodon populator", weighing in at around . Its skull, measuring upwards of in length, is the largest known skull for any machairodont.