cap 4, count 1, total 16 bytes
cap 8, count 5, total 32 bytes
cap 16, count 9, total 64 bytes
cap 32, count 17, total 128 bytes
cap 64, count 33, total 256 bytes
cap 128, count 65, total 512 bytes
cap 256, count 129, total 1024 bytes
cap 512, count 257, total 2048 bytes
cap 1024, count 513, total 4096 bytes
cap 2048, count 1025, total 8192 bytes
cap 4096, count 2049, total 16384 bytes
cap 8192, count 4097, total 32768 bytes
cap 16384, count 8193, total 65536 bytes
cap 32768, count 16385, total 131072 bytes
cap 65536, count 32769, total 262144 bytes
cap 131072, count 65537, total 524288 bytes
cap 262144, count 131073, total 1048576 bytes
cap 524288, count 262145, total 2097152 bytes
cap 1048576, count 524289, total 4194304 bytes

ほれ、やっぱり2のべき乗倍でcapacity確保されてる