In this paper, we study the impact of uncontrolled mobile conditions on signatures, when signing on two touch-screen devices of different size. 74 persons captured their signatures on both an iPad and an iPhone with different signing areas. The study exploited several quantitative indicators: an intra-personal variability measure, a statistical quality measure, and verification performance with two classifiers. We show that for 69% of writers, their signatures are more complex when made on the large surface of the iPad. This result reflects that the majority of writers are less comfortable with signing on a small handheld device with a very constrained signing surface; besides, the used rubber-tipped stylus does not offer an optimal writing precision regarding the constrained signing surface. For the remaining writers, results show that when signing on the iPad, writers tend to fill out, spontaneously, the whole available space; this leads to information loss reflected by a reduced signature complexity and stability. Performance assessment shows a significant degradation when test and reference signatures are not captured on the same platform. When test and reference signatures are captured on the same platform, performance is better when the writer signs on the small surface available on the iPhone.