> Don't people use pb so they do not have to deal with JSON?

I think that's one reason, but there are others too, like getting generated type-safe accessors and an explicit schema. I've sometimes used protobuf for this reason even when I'm not planning to use binary format.

> I'd expect that for a truly lean pb implementation, adding JSON is a 300% increase in code size?

Here is a breakdown of a binary that uses upb and links in binary format, JSON format, text format, and some generated protos:

         VM SIZE
     ---------------
      20.6%  26.5Ki    Generated Reflection
        45.7%  12.1Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/src/google/protobuf/test_messages_proto3.upbdefs.c
        39.3%  10.4Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/src/google/protobuf/test_messages_proto2.upbdefs.c
         5.0%  1.33Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/conformance/conformance.upbdefs.c
         3.1%     832    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/struct.upbdefs.c
         2.4%     648    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/wrappers.upbdefs.c
         1.2%     320    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/timestamp.upbdefs.c
         1.2%     312    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/duration.upbdefs.c
         1.1%     304    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/field_mask.upbdefs.c
         1.1%     296    [1 Others]
      19.1%  24.5Ki    JSON
        60.4%  14.8Ki    upb/json_decode.c
        39.6%  9.67Ki    upb/json_encode.c
      14.7%  18.9Ki    Reflection Runtime
        81.9%  15.4Ki    upb/def.c
        18.1%  3.41Ki    upb/reflection.c
      13.9%  17.8Ki    Generated Code
        46.9%  8.34Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/wrappers.upb.c
        17.6%  3.12Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/src/google/protobuf/test_messages_proto3.upb.c
        16.4%  2.92Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/src/google/protobuf/test_messages_proto2.upb.c
        14.7%  2.61Ki    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/descriptor.upb.c
         2.0%     356    bazel-out/k8-opt/bin/external/com_google_protobuf/conformance/conformance.upb.c
         1.4%     264    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/struct.upb.c
         0.5%      88    [2 Others]
         0.3%      48    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/any.upb.c
         0.3%      48    bazel-out/k8-opt/bin/external/com_google_protobuf/google/protobuf/duration.upb.c
      10.0%  12.9Ki    Binary Format
        54.0%  6.95Ki    upb/decode.c
        46.0%  5.92Ki    upb/encode.c
       8.8%  11.3Ki    Core Runtime
        66.3%  7.52Ki    upb/table.c
        17.8%  2.02Ki    upb/upb.c
        15.9%  1.80Ki    upb/msg.c
       5.6%  7.19Ki    [34 Others]
       3.8%  4.94Ki    Text Format
       100.0%  4.94Ki    upb/text_encode.c
       3.5%  4.46Ki    [section .rodata]
     100.0%   128Ki    TOTAL

So JSON is pulling in 25Ki directly, plus it depends on the 19Ki reflection runtime and the generated reflection.

But it is all a la carte: if you don't want JSON, just don't use it and don't link it in.

(btw that 7Ki for upb/table.c is to support protobuf maps, I want to make that a la carte also so that you don't pay for it when you don't use maps).