The main result was that for all conditions but the last one of both series (Figure 1(f) and (l)), we obtained medium-to-large effects (Cohen’s d ’s > 0.68; for A4-settings always d ’s > 0.93 indicating large effects through-out) in terms of deviations from the true value of 100%, meaning that the area size of the large rectangle was strongly underestimated. This effect was strongest when the small rectangle was centered to the large one (Cohen’s d ’s > 0.95)—and even rotating and aligning it to one single side did not dismiss this very large perceptual effect. The only way of escaping this strong visual illusion was to align two sides of both sheets at once in such a way that the small rectangle halved the large one (Figure 1(f) and (l)). This provides some indication of how the effect emerges: We seem to face a general incapability of accurately comparing more than one geometrical dimension at once. Such effects are already known from other illusions and perceptual phenomena. For instance, Piaget, Inhelder, and Szeminska (1960) already showed that (primary school) children predominantly used just one single dimension (height) to estimate volume, the so-called centration hypothesis—but see counter-evidence in later work where the integration of all three dimensions have been documented (Ebersbach, 2009). That adults are also susceptible to such volume illusions has been extensively documented, even for everyday-life objects such as typical food and drink packages (Raghubir & Krishna, 1999)—again, it seems that simple measures or single dimensions are predominantly used to estimate more complex measures or to predict the outcomes of actions by analyzing two concurrent movements (Hergovich, Grobl, & Carbon, 2011). Perceptual research, however, also offers some alternative explanations for the revealed effect. First of all, the general tendency to underestimate areas documented by psychophysical power functions with exponents below one (e.g., Ekman & Junge, 1961) cannot account for the effect seen here as the relationships between the two physical areas remained constant at 2:1 across all conditions. Assimilation theory (Pressey, 1967, 1971) appears to offer a more promising account. According to this theory, in a group of objects the ones with the extreme values, for example, lengths, will be misperceived toward the average values (the theory has many names in science, for example, “regression to the mean” in statistics or “central tendency error” in applied fields). In the given case, people would adapt their length or area assessments toward the mean of the given context lines or areas, respectively, so would underestimate the size of the bigger rectangular area. However, assimilation theory will be quite ambiguous in predicting the specific outcome for the cases of 45° rotated configurations depicted in Figure 1(e) and (k), as it is not clear how the edges will assimilate—with the smaller or longer ones? Another alternative explanation arises by comparing the area of the smaller object and the non-covered, residual of the larger rectangle. As is seen particularly clearly in the centered cases in Figure 1(a) and (g), the residual area is quite narrow, in fact much narrower than the smaller object. Indeed, we observe particularly large effects in Figure 1(a) and (b) as well as in Figure 1(g) and (h). Further qualification of such an explanation, however, reveals that the resulting underestimation of the summed up narrow areas is again an indication of problems in integrating different dimensions; we could also reconcile it with assimilation theory operating with areas instead of lengths.