Mobile apps constantly choose between REST/JSON and gRPC/Protocol Buffers for talking to a backend, but the size difference is usually described in vague percentages. This simulator builds a real API record from a configurable schema, encodes it two ways with an actual byte-accurate JSON serializer and a hand-written Protocol Buffers wire-format encoder (varint tags, length-delimited strings, fixed32 floats — the same rules real gRPC stacks use), draws every resulting byte as a colored cell in a 2D heatmap so the structural difference is visible byte-by-byte, then computes true transfer time across four cellular network profiles from bandwidth and round-trip latency. A network waterfall chart — the same visualization browser dev tools use — races both formats' transfer bars so the gap is something you watch, not just a number.