OK, I won't change it to allow additional resize while merging, after pre-resize.
But current code has two problem:

* Pre-resize happen even when resizing is not necessary. (ex. two dict has same keys).
* Pre-resize allocates too much memory which doesn't make sense.

Next patch (28509-smaller-update2.patch) seems better because:

* When pre-resize doesn't happen, at most one resize while merging.
* When pre-resize happens, no resize happens while merging.
* Doesn't make surprisingly sparse dict when two dicts have same keys.

PoC code:

import sys
b = {}
for i in range(16):
    b[i] = i
    a = b.copy()
    a.update(b)  # No need to resize
    print(i, sys.getsizeof(a), sys.getsizeof(b))

Current:

0 256 256
1 256 256
2 256 256
3 664 256
4 664 256
5 384 384  # !!!
6 664 384
7 664 384
8 664 384
9 664 384
10 664 664
11 664 664
12 1200 664
13 1200 664
14 1200 664
15 1200 664


With second patch:

0 256 256
1 256 256
2 256 256
3 256 256
4 256 256
5 384 384
6 384 384
7 384 384
8 384 384
9 384 384
10 664 664
11 664 664
12 664 664
13 664 664
14 664 664
15 664 664