Assessing feed efficiency in pasture-based systems can be challenging due to difficulties in measuring feed intake and diet preference. Furthermore, heifers may modify their intake of a particular forage species depending on its nutritive and physical attributes and on their own physiological status. For instance, heifers growing to larger mature sizes have higher maintenance requirements, which may affect their dietary preferences. The present study was designed to determine the influence of frame size (FS) on feed intake and diet selection at 2 age subclasses: postweaning, at, on average, 313 d of age (317 kg BW for larger FS and 285 kg BW for smaller FS), and as yearlings, at, on average, 403 d of age (391 kg BW for larger FS and 343 kg BW for the smaller FS). Twenty-four Angus-cross heifers were evaluated at these ages. They were housed in a drylot equipped with a Broadbent Feeding System and had ad libitum access to cubed red clover and cubed fescue hay in separate buckets. Following 3-wk acclimation periods, DMI of each forage species was assessed daily for 10 d. Body weights were also recorded daily over those 10 d. Larger and smaller FS cattle did not differ in ADG, feed conversion ratio, and Kleiber ratio at either age (P > 0.05). However, heifers from larger FS category had higher total and red clover DMI at both ages (P < 0.05) and higher proportions of red clover in their diet as yearlings (P < 0.001). Cumulative differences between clover and fescue DMI were similar at postweaning in both FS (P > 0.05); however, at yearling age, the larger FS cattle ate consistently and cumulatively more red clover (P < 0.001). The intake and diet selection of heifers across ages were not consistent for either FS category (P = 0.748), suggesting difficulties in predicting lifetime feed efficiencies based on an early measure. Although gains and feed conversion ratios were similar between FS categories, the larger FS heifers ate more. At yearling age, these heifers of larger metabolic size also selected a greater proportion of red clover in their diet. Because the FS and, thereby, BW of cattle affect intakes and diet selections, they also may impact the composition and sustainability of grasslands generally. Therefore, the relationship between FS and pasture usage likely needs to be integrated into descriptions of cow efficiency in pasture-based operations.