I used too small a size for buffers, which can cause a panic in some testing.
The new buffer size is generous and sufficient for all purposes.

Fixes #34927
Fixes #34928

Change-Id: Icdbbfed5da87fe3757be40dfd23182b37ec62d58
Reviewed-on: https://go-review.googlesource.com/c/go/+/201317Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>