BUG: groupby.sum() is inconsistent with df.sum() for large integers
- [ x ] I have checked that this issue has not already been reported.

- [ x ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd


for i in range(100):
    n = 2 ** i

    print(f"Trying n = 2 ** {i} '{n}'...")

    df = pd.DataFrame([['A', 14], ['A', n]], columns=['gb', 'val'])

    gb_sum = df.groupby('gb').sum().values[0][0]
    df_sum = df.sum().values[1]

    if gb_sum != df_sum:
        print(f"df.sum().values[1] '{df_sum}' != df.groupby('gb').sum().values[0][0] '{gb_sum}")
        break

    del df

print(f"Runing pandas {pd.__version__}")


```

#### Problem description

groupby.sum() results currently provide different results for df.sum() results for large integers.

It is expected that they should provide the same results.

#### Expected Output

When grouping by a colum with a single value, the groupby().sum() result should always equal the df.sum() result()

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.5.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.0.4
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : 0.29.16
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.3 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None
Runing pandas None

Process finished with exit code 0

</details>
