Rebuilding a complete skeleton by comparing the size and morphology of bones to those of similar, better-known species is an inexact art (though governed by some established allometric trends), and reconstructing the muscles and other organs of the living animal is, at best, a process of educated guesswork, and never perfect. Mass estimates for dinosaurs are much more variable than length estimates given the lack of soft tissue preservation in the fossilization process.
Reconstructing a complete skeleton from fossilized bones is an imprecise process (though guided by some established size-related patterns), and reconstructing the muscles and other organs of a living animal is, at best, a speculative endeavor, never perfectly accurate. Estimating the mass of dinosaurs is more variable than estimating their length due to the lack of soft tissue preservation in fossilization.