pandas to_hdf increases file size when saving twice to the same path ('write' mode, with string columns)
#### Code Sample, a copy-pastable example if possible

df = pd.DataFrame(np.random.randn(10000, 4), columns=list('ABCD'))        
                                                                          
df.to_hdf('no.string.save.once.h5', 'w')                                  
                                                                          
df.to_hdf('no.string.save.twice.h5', 'w')                                 
df.to_hdf('no.string.save.twice.h5', 'w')                                 
                                                                          
# add string columns                                                      
df2 = df.copy()                                                           
df2['E'] = 'this_is_a_string'                                             
df2['F'] = 'this is another string'                                       
                                                                          
df2.to_hdf('with.string.save.once.h5', 'w')                               
                                                                          
df2.to_hdf('with.string.save.twice.h5', 'w')                              
df2.to_hdf('with.string.save.twice.h5', 'w')                              

df2.to_hdf('with.string.save.three-times.h5', 'w')
df2.to_hdf('with.string.save.three-times.h5', 'w')
df2.to_hdf('with.string.save.three-times.h5', 'w')

#### Problem description

In the above example, df is a data frame with no string columns. It is saved once to path 'no.string.save.once.h5'; and saved twice to the path 'no.string.save.twice.h5'.  The file sizes are the same, which is what we expect:

407208  no.string.save.once.h5
407208  no.string.save.twice.h5

However, if the dataframe has string columns (e.g. df2), then saving it twice to the same path will increase the file size:

1979728  with.string.save.once.h5
2501240  with.string.save.twice.h5
3022752 with.string.save.three-times.h5

This surprises me because it seems to be saying that before I save an h5 file in "write" mode, I should really make sure that file is deleted from disk. Otherwise, if I keep writing to the same path multiple times, the file size will keep blowing up.

pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Linux
OS-release: 3.0.38-0.5-default
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C

pandas: 0.17.1
nose: 1.3.6
pip: 1.5.6
setuptools: 0.6
Cython: 0.21.1
numpy: 1.9.3
scipy: 0.16.1
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.2.2
patsy: 0.3.0
dateutil: 2.3
pytz: 2015.4
blosc: None
bottleneck: 0.8.0
tables: 3.2.2
numexpr: 2.4
matplotlib: 1.4.3
openpyxl: 2.0.4
xlrd: 0.9.0
xlwt: 0.7.4
xlsxwriter: 0.7.3
lxml: 3.4.4
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 0.9.7
pymysql: 0.6.2.None
psycopg2: None
Jinja2: None