In order to ascertain whether differing structural mechanisms could underlie blood flow restricted training (BFRT) and high intensity training (HIT), this study had two aims: (i) to gain an insight into the acute variations of muscle architecture following a single bout of two different volumes of BFRT, and (ii) to compare these variations with those observed after HIT. Thirty-five young men volunteered for the study and were randomly divided into three groups: BFRT low volume (BFRT LV), BFRT high volume (BFRT HV) and traditional high intensity resistance training (HIT). All subjects performed a bilateral leg extension exercise session with a load of 20% of one repetition maximum (1RM) in the BFRT groups, whereas the load of the HIT group was equivalent to an 85% of their 1RM. Before and immediately after the exercise bout, ultrasound images were taken from the rectus femoris (RF) and the vastus lateralis (VL). All groups increased their RF (p < 0.001) and VL (p < 0.001) muscle thickness, while the increases in pennation angle were larger in HIT as compared to BFRT LV (p = 0.013) and BFRT HV (p = 0.037). These results support the hypothesis that acute muscle cell swelling may be involved in the processes underlying BFRT induced muscle hypertrophy. Furthermore, our data indicate differing structural responses to exercise between BFRT and HIT.